GIFT  OF 


«•«*? 


ABSCISSION    OF    FLOWERS   AND    FRUITS    IN 

THE  SOLANACEAE,  WITH  SPECIAL 

REFERENCE  TO  NICOTIAN  A 


A  THESIS  SUBMITTED  IN  PAETIAL  SATISFACTION  OF 
THE  EEQUIEEMENTS  FOE  THE  DEGEEE  OF 

DOCTOE  OF  PHILOSOPHY 
AT  THE  UNIVEESITY  OF  CALIFOENIA 


BY 


JOHN  NORMAN  KENDALL 


MAY,  1917 


UNIVERSITY  OF  CALIFORNIA  PUBLICATIONS 

IN 

BOTANY 

Vol. .5,  No.  12,  pp.  347-428,  10  text  figs.,  plates  49-53  March  6,  1918 


ABSCISSION  OF  FLOWERS  AND  FRUITS  IN 

THE  SOLANACEAE,  WITH  SPECIAL 

REFERENCE  TO  NICOTIANA 


BY 


JOHN  N.  KENDALL 


UNIVERSITY  OF  CALIFORNIA  PRESS 
BERKELEY 


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Cited  as  Univ.  Calif.  PubL  Bot. 

VoL  1.      LA  Botanical  Survey  of  San  Jacinto  Mountain,  by  Harvey  Monroe 

HalL     Pp.  1-140;  plates  1-14.     June,  1902 $1.00 

2.  Two  new  Ascomycetous  Fungi  Parasitic  on  Marine  Algae,  by  Minnie 

Eeed.    Pp.  141-164;  plates  15-16.    November,  1902 .2S 

3.  Algae  of  Northwestern  America,  by  William  Albert  SetcheU  and  Na- 

thaniel Lyon  Gardner.    Pp.  165-418;  plates  17-27.    March,  1903  —    2.2S 

Vol.  2.  LA  Review  of  California  Poleinoniaceae,  by  Jessie  Milllken.  Pp.  1- 

71;  plates  1-11.  May,  1904  _ .75 

8.  Contributions  to  Cytological  Technique,  by  W.  J.  V.  Osterhout.  Pp. 

73-90;  5  text-figures.  June,  1904  „ .25 

8.  Limu,  by  William  Albert  SetcheU.    Pp.  91-113.    April,  1905  _      .25 

4.  Pest-Embryonal   Stages   of    the   Laminariaceao,    by   William   Albert 

SetcheU.     Pp.  115-138;  plates  13-14.    April,  1905  . .25 

5.  Regeneration  among  Kelps,  by  William  Albert  SetchelL    Pp.  139-168; 

plates  15-17.     July,  1905  .. .30 

6.  A  New  Genus  of  Ascomycetous  Fungi,  by  Nathaniel  Lyon  Gardner. 

Pp.  169-180;  plate  18.    July,  1905  _ .15 

7.  Teratology  in  the  Flowers  of  some  California^  Willows,  by  William 

Warner  Mott.    Pp.  181-226;  plates  16-20.    December,  1905 .60 

8.  9,  10.  11.  (In  one  cover.)     The  Resistance  of  Certain  Marine  Algae  to 

Changes  in  Osmotic  Pressure  and  Temperaturs.  The  ROle  of  Os- 
motic Pressure  in  Marine  Plants.  On  the  Importance  of  Physiolog- 
ically Balanced  Solutions  for  Plants.  The  Antitoxic  Action  of 
Potassium  on  Magnesium.  By  W.  J.  V.  Osterhout.  Pp,  227-236. 

March,   1906  _ _ _ .       3& 

12.  Cytological  Studies  in  Cyanophycea*,  by -Nathaniel  Lyon  Gardner. 

Pp.  237-296;  platea  21-26.     November,  1906  _.     LOO 

15.  On  a  Small  Collection  of  Mosses  from  Alaska,  by  J.  Cardot  and  T. 

Theriot.    Pp.  297-308;  plates  27-28.    December,  1906 .10 

14.  Some  Uureported  Alaskan  Sphagna,  together  with  a  Sun-  jury  of  the 
Cryptogamic  Work  of  the  University  of  California  Botanical  Ex- 
pedition to  Alaska  in  1899,  by  William  Albert  SetcaelL  Pp.  SOS- 
SIS.  September,  1907  _ .06 

16.  On  Nutrient  and  Balanced  Solutions,  by  W.  3,  V.  Osterhout.    Pp.  317- 

318.     October,  1907  ^ „ „ .06 

16.  A  Synopsis  of  the  North  American  Godeti&s,  by  Willis  Linn  Jepsoa. 

Pp.  319-554;  plate  29.    December,  1907  ..... AO 

Index,  pp.  355-300. 

.     1907-1009. 

L  Compositae  of  Southern  California,  by  Harvey  Monroe  Hall.    Pp.  1- 

302;  plates  1-3,  with  a  map.    December,  1907  9,00 

2,  The  Origin,  Structure,  and  Function  of  the  Polar  Caps  in  Sm&acina 

amplexicaulis  Nutk,  by  H.  D.  Densmore.     Pp.  803-330;  plates  4-8. 

December,  1908    „ .89 

8,  4.     (In  one  cover.)     The  Value  of  Sodium  to  Plants  by  Reason  of  It» 

Protective  Action.     On  the  Effects  of  Certain  Poisonous  Gases  on 

Plants.    By  W.  J.  V.  Ostsrhout.    Pp.  331-340.    June,  1908  .„ _       .10 

6.  Contributions  to  the  BInowledge  of  the  California  Species  of  Cmsta- 

CCOBS  Corallines.  L  by  Maurice    Barstow  Nlchola.     Pp.  341-348; 

plate  9.     December,  1908  ..  .10 


UNIVERSITY  OF  CALIFORNIA   PUBLICATIONS 

IN 

BOTANY 

Vol.  5,  No.  12,  pp.  347-428,  10  text  figs.,  plates  49-53  March  6,  1918 


ABSCISSION  OP  FLOWERS  AND  FRUITS  IN 

THE  SOLANACEAE,  WITH  SPECIAL 

REFERENCE  TO  NICOTIAN  A 

BY 

JOHN  N.  KENDALL 


CONTENTS 

PAGE 

I.  Introduction    348 

II.  Summary  of  the  literature  350 

III.  Technique    361 

IV.  Histology  and  cytology  of  the  pedicel  363 

1.  Histological  and  cytological  condition  of  the  mature  pedicel 363 

2.  Development  of  the  separation  zone  in  Nicotiana  and  Lycoper- 
sicum  367 

3.  Increase  in  size  and  development  of  mechanical  tissue  in  the 
pedicel  of  Nicotiana  and  Lycopersicum  369 

V.  The  process  of  abscission  371 

1.  General  description  of  the  process  in  several  genera 371 

2.  Method  of  cell  separation  376 

VI.  Abscission  of  the  style  and  corolla  383 

VII.  Time  of  abscission  385 

1.  Keaction  time  385 

2.  Abscission  time  396 

VIII.  Experimental  induction  of  abscission  397 

1.  Induction  by  illuminating  gas  397 

2.  Action  of  acids  on  the  separation  layer  of  Nicotiana  404 

3.  Induction  by  mechanical  injury  406 

4.  The  ability  of  certain  species  to  throw  off  pedicels  from  which 

all  the  floral  organs  have  been  removed,  as  related  to  the  induc- 
tion of  abscission  by  mechanical  injury  410 

IX.  Summary   411 

X.  Conclusion  415 

XI.  Literature  cited  418 

XII.  Plates  ..  420 


374794 


- 

?  *•;  •  :  %«• *  / 
.  •  •»•«• 

348  University  of  California  Publications  in  Botany         [VOL.  5 


INTRODUCTION 

Although  it  is  a  matter  of  common  observation  that  many  plants 
are  capable  of  detaching  portions  of  the  body,  the  underlying  cause 
and  the  actual  mechanism  which  bring  about  such  separation  are 
only  slightly  understood.  The  process  has  often  been  described  as 
one  of  self-pruning  by  which  the  plant  rids  itself  of  useless  portions 
of  its  body.  Since  abscission  is  sometimes  confused  with  exfoliation, 
it  seems  desirable  here  to  distinguish  definitely  between  these  two 
phenomena.  It  can  be  said  that,  in  general,  exfoliation  is  preceded 
by  drying  and  death  of  the  part  to  be  cast  off  and  that  actual  separa- 
tion of  the  organ  is  accomplished  by  a  mechanical  break  through  dry, 
dead  tissues.  Abscission,  on  the  other  hand,  is  usually  not  preceded 
by  drying  and  death  of  the  organ  concerned  and  its  detachment  is 
accomplished  by  a  separation  along  the  plane  of  the  middle  lamellae  of 
active  living  cells. 

Abscission  may  be  either  axial  or  lateral.  Axial  abscission  includes 
the  abscission  of  portions  of  stems,  shoots,  entire  flowers  or  fruits. 
Lateral  abscission  includes  the  abscission  of  leaves,  petioles,  sepals, 
petals  or  styles.  Considerable  attention  has  been  given  by  investi- 
gators to  the  abscission  of  flowers  because  of  the  theoretical  detriment 
to  crops  caused  by  the  fall  of  the  flower  before  the  fruit  is  formed. 

The  cause  of  leaf-fall  in  deciduous  species  is  connected  with  peri- 
odic changes  in  the  physiological  condition  brought  about  by  changes 
in  the  environment.  In  the  case  of  some  herbaceous  plants  and  occa- 
sionally in  trees,  sudden  changes  in  environmental  conditions  result- 
ing in  a  loss  of  physiological  equilibrium  often  cause  the  throwing 
off  of  leaves,  flowers  or  even  small  shoots.  In  certain  species,  any- 
thing which  tends  to  loss  or  completion  of  function  within  or  peculiar 
to  an  organ  causes  the  organ  to  be  thrown  off.  Thus,  staminate  flow- 
ers are  commonly  thrown  off  soon  after  anthesis  and  pistilate  flowers 
generally  fall  when  fertilization  is  prevented.  Similarly,  certain 
species — e.g.,  Impatiens  Sultani  and  Mirabilis  Jalapa — throw  off  por- 
tions of  their  stems  which  have  been  rendered  useless  as  a  part  of  the 
conducting  system  because  of  injury  or  removal  of  distal  buds  or 
leaves. 

The  following  definitions  of  terms,  which  will  be  used  throughout 
this  paper,  are  made  necessary  because  of  a  notable  lack  of  uniformity 
in  their  usage  by  various  investigators  who  have  dealt  with  abscission. 


1918]    Kendall:  Abscissi-on  of  Flowers  and  Fruits  in  Solanaceae    349 

1.  Abscission  is  the  detaching  of  an  organ  by  the  separation  of 
actively  living  cells  at  or  near  its  base. 

2.  The  separation  layer  (Mohl's  Trennungschichte)  is  the  layer  of 
cells  the   components  of  which   will   separate   from   one   another   at 
abscission. 

3.  The  separation  cells  or  absciss  cells  are  the  cells  that  make  up 
the  separation  layer. 

4.  The  separation  zone  is  the  general  region  through  which  abscis- 
sion takes  place  and  usually  is  largely  proximal  to  the  separation  layer. 

A  preliminary  account  of  abscission  in  Fx  species  hybrids  of  Nico- 
tiana  has  already  appeared  (Goodspeed  and  Kendall,  1916).  The 
present  study  represents  an  amplification  of  this  investigation  and  its 
extension  to  other  species  of  the  Solanaceae.  It  is  particularly  con- 
cerned with  the  following:  (1)  the  position  of  the  separation  layer; 
(2)  the  origin  of  the  separation  layer;  (3)  the  cytology  of  the  separa- 
tion layer;  (4)  the  process  of  abscission,  including  (a)  a  description 
of  the  appearance  of  the  separation  layer  in  consecutive  stages  of  the 
process  and  (b)  the  method  of  cell  separation;  (5)  the  time  occupied 
by  abscission,  including  (a)  the  time  between  the  application  of  the 
stimulus  and  fall  (reaction  period)  and  (b)  the  time  involved  in  the 
actual  process  of  cell  separation  (abscission  period)  ;  (6)  experimental 
induction  of  abscission. 

Although  the  investigation  reported  here  is  largely  a  morpholog 
ical  one,  the  results  of  the  experiments  on  the  method  of  cell  separa- 
tion, the  time  of  abscission  and  the  induction  of  abscission  seem  to 
have  a  distinct  physiological  significance  as  well. 


350  University  of  California  Publications  in  Botany         [VOL.  5 

SUMMARY  OF  THE  LITERATURE 

Since  the  literature  on  abscission  is  rather  voluminous,  it  seems 
best  to  present  the  following  discussion  under  several  different  head- 
ings corresponding,  to  a  certain  extent,  with  the  six  main  topics  of 
interest  mentioned  in  the  introduction.  The  summary  below  is  largely 
confined  to  the  literature  on  axial  abscission,  although  that  on  lateral 
abscission  is  considered  in  so  far  as  it  has  a  direct  bearing  on  the  most 
important  aspects  of  the  abscission  problem. 

1.  HISTOLOGY  OF  THE  PEDICEL 

a.  POSITION  OF  THE  SEPAEATION  LAYER 

Hoehnel  (3880),  discussing  the  fall  of  catkins  in  Populus  and 
Salix,  locates  the  separation  layer  at  the  base  of  the  catkin.  The  gen- 
eral region  at  the  base  of  the  catkin,  in  the  distal  part  of  which  the 
separation  layer  is  located,  he  calls  the  "separation  zone."  In  Salix, 
actual  separation  occurs  in  the  separation  layer,  but  in  Populus  it 
occurs  in  the  parenchyma  entirely  outside  the  separation  layer. 
According  to  Balls  (1911),  the  separation  layer  in  the  cotton  flower 
is  located  at  the  base  of  the  pedicel.  The  layer  is  located  by  Hannig 
(1913)  at  the  base  of  the  pedicel  in  Nicotiana  Tabacum,  N.  rustica, 
N.  accuminata,  N.  sylvestris,  Datura,  and  Atropa,  and  at  the  tip  of 
the  pedicel  in  Nicotiana  Langsdorffii,  Salvia  Aloe,  Cuphea,  and  Gasteria. 
He  finds  it  occurring  at  the  middle  of  the  pedicel  in  Impatiens  Sultani, 
Solanum  tuberosum,  Lycopersicum,  Asparagus,  and  Begonia.  Gort- 
ner  and  Harris  (1914)  and  Lloyd  (1914&),  working  on  the  abscission 
of  internodes  as  the  result  of  injury  in  Impatiens  Sultani,  locate  the 
separation  layer  at  the  first  node  below  the  injury  and  just  above  the 
axillary  bud.  Occasionally,  according  to  the  latter  investigators,  ab- 
scission may  occur  at  the  second  or  third  node  below  the  injury  and 
in  these  cases  the  buds  at  the  first  or  second  nodes  seem  to  be  abortive. 

The  separation  layer,  according  to  Hannig  (1913),  may  occur  at 
the  base  of  the  complete  inflorescence  in  Impatiens  and  Oxybaphus. 
According  to  Lloyd  (1914a),  the  separation  layer  occurs  at  the  base 
of  the  pedicel  in  cotton  and  at  the  base  of  the  ripened  ovary  in  grape 
"shelling."  In  the  abscission  of  internodes  and  tendrils  in  Vitis  and 
Ampelopsis,  Lloyd  (19140)  locates  the  layer  near  but  not  exactly  at 
the  base  of  the  internode.  A  peculiar  case  illustrating  the  result  of 
displacement  of  the  stem  on  the  location  of  the  separation  layer  is 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaveae    351 

discussed  by  Lloyd  (1914a)  for  Ampelopsis  and  Gossypium.  In  the 
latter,  abscission,  in  the  abnormal  case,  occurred  down  the  internode 
at  the  base  of  the  pedicel.  This  is  explained  as  the  result  of  a  dis- 
placement during  growth  by  which  part  of  the  pedicel  becomes  united 
to  the  stem. 

Occasionally,  grooves  or  swellings  are  noticed  at  the  base  of  the 
organ  being  abscissed  where  they  correspond  more  or  less"  exactly  to 
the  general  position  of  the  separation  layer.  Examples  are  given  by 
Hannig  (1913)  for  Lycopersicum  and  Solanum  tuberosum  and  by 
Balls  (1911)  for  Gossypium.  Abscission  may  occasionally  occur, 
according  to  Lloyd  (1914a),  above  a  small  bract.  According  to  these 
latter  investigators,  there  is  more  often  no  external  indication  of  the 
layer.  Frequently,  grooves  bear  no  relation  to  the  layer  because  in 
many  cases  of  this  kind  (Hannig,  1913,  for  Brunfelsia)  separation 
occurs  a  short  distance  distal  to  the  groove. 

From  the  above  brief  summary  it  is  evident  that  in  the  case  of 
axial  abscission  the  separation  layer  is  located  at  or  near  the  base  of 
an  internode.  Apparent  exceptions  are  reported  by  Hannig  (1913) 
in  which  it  is  seemingly  located  at  the  middle  of  an  internode.  It 
seems  probable  that  a  more  critical  re-examination  might  reveal  the 
fact  that  even  these  exceptions  accord  with  the  general  rule.  In  these 
cases,  for  example,  the  pedicel  of  the  flowers  in  question  might  be 
composed  of  two  internodes. 

6.  ORIGIN  OF  THE  SEPAEATION  LAYEE 

Kubart  (1906)  states  that  the  occurrence  of  the  separation  layer 
in  all  tyes  of  abscission  may  be  explained  in  one  of  the  three  following 
ways:  (a)  the  separation  layer  is  preformed  and  represents  simply 
a  portion  of  the  primary  meristem  which  has  remained  in  its  original 
active  state;  (6)  it  represents  a  secondary  meristem;  (c)  the  primary 
meristem  may  function  directly  as  a  separation  layer.  The  differ- 
ence between  a  and  c  is  only  a  difference  in  time,  c  being  added  to 
explain  the  origin  of  the  separation  layer  in  abscission  of  very  young, 
embryonic  tissues.  In  a,  the  separation  layer  is  present  at  the  base 
of  the  organ  from  the  start  of  its  development,  but  in  &  this  layer  has 
to  be  formed  by  a  secondary  meristem  before  abscission  can  occur.  In 
a,  cell  divisions  are  not  normally  found  preceding  abscission,  but  in  & 
and  c  they  are.  Mohl  (1860),  working  on  the  fall  of  the  flower  in 
Aesculus,  Pavia,  Lagenaria,  Cucumis,  and  Ricinus,  states  that  the 
separation  layer  in  these  forms  is  of  type  6.  Throughout  his  entire 


352  University  of  California  Publications  in  Botany         [V°L.  5 

work  Mohl  gives  the  general  impression  that  it  is  necessary  for  a  sep- 
aration layer  to  be  formed  from  a  secondary  meristem  before  abscis- 
sion can  occur.  Wiesner  (1871),  working  on  leaf -fall  in  general, 
observes  that  the  separation  layer  is  not  generally  of  type  b,  as  Mohl 
believes,  but  more  often  of  type  a.  According  to  Becquerel  (1907), 
the  separation  layer  is  formed  in  the  pedicel  of  Nicotiana  from  a  sec- 
ondary meristem  (type  b).  In  the  cotton  flower  Balls  (1911)  finds 
that  the  separation  layer  is  of  type  Z>,  but  according  to  Lloyd  (1914a 
and  1916&)  there  is  doubt  as  to  this  conclusion,  since  in  the  case  of 
very  young  cotton  flowers  in  which  abscission  occurs  very  suddenly, 
he  finds  only  rarely  that  cell  divisions  do  not  precede  abscission. 
Hannig  (1913),  for  flower-vfall  in  general,  states  that  a  separation 
layer  of  type  a  is  always  present  but  in  certain  species  a  secondary 
layer  of  type  b  may  also  be  formed,  through  which  separation  may  or 
may  not  occur.  Hannig,  differing  from  Becquerel  ( 1907 ) ,  points  out 
that  the  separation  layer  in  Nicotiana  is  of  type  a.  Lloyd  (1914a) 
and  Loewi  (1907)  indicate  that  in  general  a  layer  of  cells  through 
which  abscission  is  possible  is  more  often  of  type  a  than  of  type  b. 
They  believe,  however,  that  the  separation  layer  is  not  a  definite 
morphological  structure  but  represents  merely  a  physiological  con- 
dition. 

c.  CYTOLOGY  OF  THE  SEPAKATION  LAYEE 

Mohl  (1860)  describes  the  separation  cells  in  the  flower  stalk  as 
young,  active,  small  cells  which  generally  contain  no  starch.  He  also 
states  that  in  most  cases  cell  divisions  are  characteristic  of  the  sep- 
aration layer,  i.e.,  that  the  separation  layer  is  meristematic.  Hoehnel 
(1880)  finds  that  cell  divisions  are  characteristic  of  the  proximal  por- 
tion of  the  separation  zone  in  Salix  and  Populus  but  in  the  distal 
portion,  where  the  separation  layer  is  developed,  these  divisions  are 
not  so  numerous.  In  some  cases  he  finds  separation  taking  place  in 
the  parenchyma,  entirely  outside  the  "zone"  where  there  had  been 
no  cell  divisions.  The  separation  cells  in  Nicotiana  are  described  by 
Becquerel  (1907)  as  small,  practically  undifferentiated  cells  with 
large  nuclei.  In  Begonia,  Fuschia,  Mirabilis,  and  Impatiens  Hannig 
(1913)  describes  the  tissue  as  secondary  meristem  (type  b)  with  the 
cells  rectangular  in  shape  and  arranged  in  more  or  less  definite  rows. 
In  contrast  to  the  above  observations,  he  describes  the  cells  as  small, 
irregularly  arranged  and  spherical  in  Salvia,  Solanum  nigrum,  and 
Nicotiana  Tabacum.  In  Solanum  nigrum  the  separation  layer  consists 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    353 

of  two  or  three  tiers  of  cells  but  in  N.  Tabacum  the  layer  is  made  up 
of  ten  to  fifteen  tiers. 

Hannig  (1913),  by  means  of  various  microchemical  tests,  can 
detect  no  chemical  difference  between  the  cell  walls  of  the  separation 
layer  and  those  of  the  cells  on  either  side.  Lloyd  (1914a),  however, 
claims  that  the  cell  walls  of  the  separation  cells  break  down  more 
quickly  when  treated  with  caustic  potash  than  do  the  walls  of  normal 
cells.  Starch  grains  are  frequently  noted  by  Hannig  and  Lloyd 
(1916a)  as  occurring  in  the  separation  cells,  especially  in  the  abscis- 
sion of  internodes  by  Mirabilis  Jalapa. 

An  examination  of  the  literature  thus  makes  it  evident  that  there 
has  been  a  great  difference  noted  in  the  various  species  in  regard  to 
the  character  of  the  separation  cells.  The  one  characteristic  of  these 
cells,  however,  to  which  there  is  no  exception  is  that  they  are  in  an 
actively  living  condition. 

2.  THE  PROCESS  OF  ABSCISSION 

a.  METHODS  OF  ABSCISSION 

It  has  been  found  that  in  practically  all  cases  of  abscission  the 
detaching  of  the  organ  is  brought  about  by  the  separation  of  cells 
along  the  plane  of  the  middle  lamella.  It  is  the  method  noted  by 
Mohl  (1860),  Wiesner  (1871),  and  Kubart  (1906),  who  call  it  a  pro- 
cess of  maceration.  Correns  (1899)  calls  it  a  process  of  ' '  schizolysis. " 
Correns,  however,  in  the  same  work  describes  a  new  and  different 
method  of  abscission  (rhexolysis)  which  he  finds  in  mosses.  In  this 
latter  method,  separation  is  accomplished  by  a  seemingly  passive 
break  of  tissues  irrespective  of  the  position  of  cell  walls.  This  may 
be  the  case  in  the  style  of  cotton  (cf.  Lloyd,  1914&).  This  same 
method  has  been  reported  by  Tison  (1900)  in  the  leaf  of  Aristolochia 
Sipho,  although  the  evidence  has  been  called  in  question  by  Lloyd 
and  Loewi  (1907).  Still  another  type  of  abscission  has  been  described 
by  Hannig  (1913)  as  a  result  of  experiments  on  Mirabilis  and  Oxy- 
baphus.  In  these  plants  he  finds  separation  being  brought  about  by 
a  disorganization  and  dissolving  away  of  a  complete  tissue.  Lloyd 
(1916a),  on  the  other  hand,  states  that  separation  in  these  species  is 
accomplished  by  cell  separation  and  is  thus  true  schizolysis.  Hannig 
was  doubtless  confused  in  this  case  by  the  cell  elongations  which 
Lloyd  observes  and  by  which  the  membranes  surrounding  the  proto- 
plasts are  drawn  out  exceedingly  thin.  Loewi  (1907),  working  on 


354  University  of  California  Publications  in  Botany         [VOL.  5 

several  genera,  including  Cinnamomum  and  Euonymus,  notes  and 
figures  cell  elongations  similar  to  those  figured  by  Lloyd  (1916a). 
These  cell  elongations  he  finds  so  frequent  and  conspicuous  that  he 
proposes  a  distinct  type  of  abscission,  calling  it  a  "  Schlauchzell 
raechanismus. ' ' 

Loewi,  on  the  basis  of  his  studies,  seeks  to  classify  the  methods  of 
cell  separation  in  abscission  under  six  different  headings,  which  per- 
haps would  be  more  appropriately  presented  under  the  next  subject 
of  consideration  (the  methods  of  cell  separation)  ;  but  since  the  author 
gave  them  as  distinct  methods  of  abscission  they  will  be  considered 
here.  They  are:  (1)  "round  cell"  mechanism;  (2)  dissolving  of  the 
middle  lamella;  (3)  maceration;  (4)  turgescence;  (5)  cell  elonga- 
tions; (6)  "hard  cell"  mechanism.  They  are  to  be  considered  merely 
as  factors  which,  singly  or  in  combinations,  may  enter  in  as  a  part  of 
the  normal  process  of  cell  separation.  Loewi  also  claims  that  by  con- 
trolling the  temperature,  humidity,  and  various  other  factors  sur- 
rounding the  plant  he  can  influence  it  to  such  an  extent  as  to  change 
its  method  of  cell  separation. 

ft.  METHOD  OF  CELL  SEPARATION 

It  has  been  held  by  various  investigators  that  the  cell  separation, 
almost  universally  connected  with  abscission,  can  be  caused  either  by 
(a)  chemical  alteration  and  dissolving  of  the  middle  lamella  or  by 
(&)  increase  in  cell  turgor.  This  whole  matter  has  received  consider- 
able attention,  although  very  little  direct  evidence  has  been  obtained. 
Wiesner  (1871  and  1905)  states  that  cell  separation  is  caused  by  the 
dissolution  of  the  middle  lamella  and  by  increased  turgor.  Kubart 
(1906)  and  Loewi  (1907)  agree  entirely  with  Wiesner  on  this  point. 
Strasburger  (1913),  Tison  (1900),  Lee  (1911),  Hannig  (1913),  and 
Lloyd  (1916#  and  b)  believe  that  cell  separation  is  accomplished  by 
the  dissolution  of  the  middle  lamella.  Practically  all  investigators 
have  noticed  the  turgid  appearance  of  the  cells  after  separation, 
although  this  of  course  does  not  constitute  evidence  that  the  separa- 
tion is  due  to  increased  turgor.  Fitting  (1911)  claims  that  the  sep- 
aration is  accomplished,  at  least  in  some  cases,  solely  by  an  increased 
turgor  of  the  separation  cells.  He  bases  his  claim  on  the  fact  that 
abscission  is  very  often  too  rapid  to  allow  time  for  the  dissolution  of 
the  middle  lamella.  He  also  mentions  the  fact  that  the  separation 
cells  are  very  often  small,  spherical  cells,  the  type  of  cell  which  would 
respond  most  readily  by  an  increase  in  cell  turgor.  On  account  of  its 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanac&ae    355 

rapidity  and  regularity  of  reaction,  Fitting  claims  that  abscission  is 
a  semi-tropistic  phenomenon  and  suggests  the  term  "Chorismus"  to 
designate  this  type  of  reaction. 

It  has  been  observed  by  Hannig  and  Fitting  that  the  presence  of 
various  narcotic  vapors  in  the  atmosphere  around  certain  species  of 
plants  causes  their  flowers  or  merely  the  petals  to  be  thrown  off. 
Various  aspects  of  this  general  problem  of  the  reaction  ofplant  tissues 
to  such  agencies  have  been  investigated.  It  has  been  determined  by 
various  plant  physiologists  that  the  presence  of  narcotic  vapors,  such 
as  illuminating  or  acetylene  gas,  in  the  air  around  certain  plant  tissues 
causes  the  proportion  of  soluble  carbohydrates  within  their  cells  to 
increase.  This  increase  in  the  amount  of  soluble  carbohydrates  would 
indicate  an  increase  in  cell  turgor.  The  question  at  once  arises, 
whether  or  not  this  increase  in  turgor  can  effect  complete  separation 
or  maceration  of  cells  without  the  occurrence  of  chemical  alteration  in 
the  walls.  Kichter  (1908)  resting  his  case  on  experimental  evidence, 
throws  some  light  on  this  problem.  Various  kinds  of  plant  tissues 
which  he  subjected  to  acetylene  vapors  broke  in  pieces  because  of  the 
maceration  and  collapse  of  the  living  cells  within.  He  finds  that  in 
the  case  of  the  cells  of  tissues  which  are  commonly  rich  in  starch 
inclusions,  such  as  the  fruit  of  the  snowberry  and  the  potato  tuber, 
the  maceration  is  most  complete.  In  the  potato,  for  example,  3  to 
5  mm.  of  material  on  the  surface  become  completely  macerated  after 
being  subjected  to  acetylene  gas.  According  to  Bichter  and  Grafe 
(1911),  the  proportion  of  sugar  in  starchy  seedlings  subjected  to 
acetylene  gas  is  larger  than  in  seedlings  grown  under  normal  condi- 
tions. In  seedlings  from  oily  seeds,  however,  the  amount  of  sugar  is 
decreased  and  the  proportion  of  glycerine  and  fatty  acids  increased. 
The  conclusion  is  therefore  drawn  that  the  subjection  of  plant  tissues 
to  narcotic  vapors  favors  the  hydrolysing  process  in  the  cells  involved. 
The  work  of  these  two  investigators  goes  to  show  that  narcotic  vapors 
may  cause  abscission  by  acting  in  either  of  the  most  important  meth- 
ods suggested  as  responsible  for  cell  separation ;  they  may  increase  cell 
turgor  on  the  one  hand  or  favor  the  hydrolysis  of  the  middle  lamella 
on  the  other. 

Lloyd  (1916a)  presents  evidence  of  chemical  change  in  the  cell 
walls  of  the  separation  layer  before  abscission.  These  cell  walls  stain 
in  the  usual  manner  with  iodine,  giving  a  light  brownish  color,  but 
as  abscission  commences,  they  give  a  faint  blue  color  when  stained 
with  iodine  and  washed  out  with  water.  Shortly  before  cell  separa- 


356  University  of  California  Publications  in  Botany         [VOL.  5 

tion  commences,  Bisrnark  brown  and  Ruthenium  red  fail  to  stain  the 
primary  and  secondary  cellulose  membranes  of  the  separation  cells, 
although,  when  abscission  does  not  occur,  the  entire  cell  wall  is  stained 
in  the  normal  manner.  The  cells  when  separating  seem,  furthermore, 
to  be  surrounded  only  by  the  thin  tertiary  membranes.  Lloyd,  in  his 
work,  figures  cells  in  the  process  of  separation  which  show  the  disso- 
lution of  the  primary  and  secondary  membranes  of  the  cell  wall. 

Various  interpretations  are  given  to  the  repeatedly  observed 
occurrence  of  cell  divisions  preceding  and  accompanying  abscission 
Mohl  (1860)  expresses  the  opinion  that  cell  divisions  are  generally 
necessary  before  abscission  can  occur.  Investigators  since  his  time 
have  disproved  the  universal  occurrence  of  cell  divisions  because  they 
find  more  and  more  cases  where  no  cell  divisions  occur.  Lloyd 
(1914a)  maintains  that  cell  divisions  are  not  of  necessity  correlated 
with  abscission  but  are  merely  evidences  of  renewed  growth  and 
wound  responses.  As  evidence  he  states  that  cell  divisions  are  some- 
times absent  and  sometimes  present  in  the  same  species.  He  cites 
(19165)  the  cotton  plant  as  a  typical  example  in  which  cell  divisions 
are  present  in  the  abscission  of  older  flowers  in  which  the  reaction  to 
stimulus  is  slow.  In  young  flowers  and  flower  buds  abscission  may 
proceed  without  cell  division.  He  further  notes  (19140)  that  cell 
divisions  sometimes  precede  and  at  other  times  follow  abscission  in  a 
given  species. 

c.  AGENCIES  ACTIVE  IN  BEINGING  ABOUT   THE  DISSOLUTION 
OP  THE  MIDDLE  LAMELLA 

Very  few  theories  have  been  proposed  to  account  for  the  dissolu- 
tion of  the  middle  lamella  and  practically  no  evidence  of  any  kind 
has  been  submitted.  "Wiesner  (1905)  claims  that  in  leaf -fall  an 
organic  acid,  produced  as  a  result  of  lessening  of  cell  activity  and 
stagnation  of  cell  contents,  acts  on  the  middle  lamella.  His  evidence 
for  this  statement  has  to  do  with  obtaining  acid  reactions  with  litmus 
from  cells  at  the  base  of  the  petiole  during  abscission.  Kubart  (1906) 
also  obtains  acid  reactions  at  the  base  of  the  corolla  in  Nicotiana  dur- 
ing abscission  and,  although  agreeing  with  Wiesner  that  an  organic 
acid  probably  causes  the  dissolution  of  the  middle  lamella,  he  also 
admits  the  possibility  that  an  enzyme  plays  a  part  in  the  process. 
Lloyd  (1916&)  makes  the  statement  that  the  dissolution  of  the  middle 
lamella  is  a  process  of  hydrolysis  and  although  making  no  definite 
statement  on  the  subject  appears  to  take  it  for  granted  that  an 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    357 

enzyme  of  some  kind  is  the  active  factor.  Indeed,  since  all  hydrolys- 
ing  processes  of  living  cells  are  now  supposed  to  be  due*  to  the  action 
of  enzymes,  there  is  no  reason  to  suppose  that  the  hydrolysis  of  the 
middle  lamella  does  not  conform  to  the  general  rule.  For  it  is  known 
that  an  enzyme,  pectosinase,  is  capable  of  breaking  down  the  pectose 
of  which  the  middle  lamella  is  composed.  However,  until  more  is 
known  concerning  the  nature  of  this  particular  enzyme  ~it  remains 
impossible  to  get  more  definite  evidence  on  this  phase  of  the  problem. 

3.  ABSCISSION  OF  THE  COROLLA 

Reiche  (1885)  gives  an  account  of  the  fall  of  the  corolla  in  a 
large  number  of  species  belonging  to  about  forty-five  families  of  the 
monocotyledons  and  dicotyledons.  He  finds  that  the  corolla  may  be 
thrown  off  in  one  of  three  different  ways:  (1)  by  the  activity  of  a 
small-celled  separation  layer;  (2)  through  decay;  (3)  through  in- 
crease in  size  of  the  ovary,  thus  tearing  off  the  tissue  involved  at 
the  base  of  the  corolla.  In  many  cases  of  true  abscission — case  1 
above — Reiche  finds  that  the  separation  layer  is  preformed  and  ready 
to  function  at  any  moment.  This  represents  a  contradiction  of 
Mohl's  observations,  according  to  which  the  fall  of  the  corolla  is 
usually  due  to  the  action  of  a  separation  layer  formed  shortly  before 
fall.  According  to  Reiche,  the  separation  layer  is  very  seldom  morpho- 
logically differentiated  from  the  neighboring  tissue,  but  in  a  few  cases 
he  describes  the  separation  layer  as  consisting  of  a  layer  of  cells 
smaller  than  the  neighboring  cells  on  either  side. 

Kubart  (1906),  in  his  account  of  abscission  of  the  corolla  in  sev- 
eral different  species,  describes  and  figures  the  process  which  takes 
place  in  Nicotiana.  The  separation  layer  in  this  genus  he  finds  to  be 
in  no  way  morphologically  differentiated,  of  indefinite  shape,  and 
located  about  1  mm.  above  the  base  of  the  corolla  tube.  In  this  gen- 
eral region  a  large  number  of  cells  separate  from  one  another,  all  the 
cells  in  cross-section  taking  part  except  the  epidermal  cells  and  the 
tracheae.  Fitting  (1911),  in  his  work  on  the  shedding  of  petals,  de- 
scribes the  process  of  abscission  in  several  genera,  paying  particular 
attention  to  Erodium,  Geranium,  Linum,  Helianthemum,  Perlagonium, 
and  Verbascum.  Separation  in  these  cases  takes  place  through  a 
region  of  small,  spherical  cells  rich  in  protoplasm.  The  separation 
layer  is  not  sharply  differentiated  as  compared  with  the  tissues  on 
either  side  but  is  located  in  a  restricted  region  at  the  base  of  the  petal. 


358  University  of  California  Publications  in  Botany         [VOL.  5 

He  finds  no  cell  divisions  preceding  or  accompanying  abscission.  The 
process  in  premature  abscission  he  finds  differing  in  no  way  from 
that  in  normal  abscission  after  fertilization.  These  conditions,  he 
states,  correspond  more  or  less  to  those  which  he  finds  in  the  pedicel 
during  flower-fall. 

4.  TIME  OF  ABSCISSION 

The  time  elapsing  between  anthesis  and  flower-fall  in  partially 
sterile  F1  species  hybrids  of  Nicotiana  and  between  emasculation  at 
anthesis  and  fall  in  the  case  of  their  corresponding  parents  is  dis- 
cussed in  a  previous  paper  (Goodspeed  and  Kendall,  1916).  It  was 
there  stated  that  the  average  time  is  about  nineteen  days  in  Fl  H154, 
seven  in  Fx  H179,  five  in  N.  Tabacum  var.  macrophylla,  and  thirteen 
in  N.  sylvestris.  When  we  turn  to  the  question  of  the  reaction  time 
in  premature  abscission  occurring  before  the  normal  time  as  the  result 
of  sudden  changes  in  external  environmental  conditions,  we  find  that 
this  subject  has  received  only  slight  attention.  According  to  Lloyd 
(1914a),  the  cotton  "square"  falls  in  one  to  twenty-two  days  after 
the  weevil  lays  its  eggs,  the  average  time  being  eight  days.  In  one 
experiment  in  which  the  ovary  was  cut  transversely,  Lloyd  was  able 
to  cause  one  hundred  per  cent  of  the  young  bolls  to  fall  in  forty-eight 
hours  and  ninety  per  cent  in  twenty-four  hours.  Larger  bolls  take  a 
longer  time  to  respond  to  injury  than  do  smaller  ones,  as  a  result  of 
the  development  of  the  pedicel  to  a  condition  in  which  abscission 
meets  greater  resistance.  Cotton  * '  squares, ' '  he  finds,  take  a  longer  time 
to  respond  than  young  bolls,  the  former  shedding  thirty-five  to  sixty 
per  cent  in  thirty-six  hours  and  the  latter  forty  to  seventy  per  cent  in 
forty-eight  hours.  On  the  other  hand,  he  obtains  no  evidence  (19166) 
that  the  reaction  times  are  any  shorter  in  small  buds  than  in  larger 
ones.  The  reaction  times  in  cases  where  the  injury  is  performed  in  the 
evening  seem  to  be  shorter  by  about  twelve  hours  than  in  cases  where 
the  injury  is  performed  in  the  morning.  This  difference  he  ascribes 
to  the  increase  in  turgidity  which  takes  place  during  the  night  and 
which  serves  to  hasten  the  reaction.  Very  severe  injuries  to  the  ovary, 
he  finds,  cause  fall  of  young  bolls  quicker  than  less  severe  injuries. 
Injuries  which  are  less  severe  than  those  mentioned  above  and  per- 
formed so  as  to  imitate  the  injury  inflicted  on  the  ovary  by  insect 
larvae  caused  shedding  in  three  to  six  days,  with  most  of  the  fall 
occurring  on  the  fifth  day.  Summing  up  his  entire  results,  Lloyd 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    359 

(1916&)  states  that  under  field  conditions  the  responses  to  all  kinds 
of  stimuli  conducive  to  abscission  become  evident  within  ten  days, 
with  the  maximum  frequency  below  six  days. 

The  actual  time  involved  in  the  process  of  abscission  (abscission 
time)  has  received  even  less  attention  than  the  problems  discussed 
above.  Fitting  (1911)  states  that  abscission  time  may  occasionally 
be  very  short,  forty-five  seconds  to  five  minutes  in  the  petals  of  Ver- 
bascum  and  thirty  seconds  to  six  minutes  in  Geranium.  Lloyd  (1914a 
and  19166)  finds  abscission  after  injury  of  the  small  cotton-boll  taking 
place  within  four  hours,  the  length  of  time  depending  somewhat  on 
the  age  of  the  boll.  In  a  previous  paper  (Goodspeed  and  Kendall, 
1916)  a  general  estimate  of  the  abscission  time  was  given  and  it  was 
stated  that  normal  abscission  due  to  lack  of  fertilization  takes  place 
in  Nicotiana  hybrids  in  four  to  eight  hours  and  premature  abscission 
in  one  to  four  hours. 

5.  EXPERIMENTAL  INDUCTION  OF  ABSCISSION 

According  to  Hannig  and  Loewi,  abscission  may  be  induced  in 
two  different  ways.  First  by  abnormal  external  conditions  ("spon- 
taneous" or  premature  abscission)  and  second  by  normal  internal 
conditions  at  the  normal  time  ("automatic"  or  normal  abscission). 
We  shall  consider  in  the  following  summary  of  the  literature  only 
two  aspects  of  induction  of  the  first  type. 

a,  INDUCTION  BY  NAECOTIC  VAPOES 

Hannig  (1913)  reports  a  comparative  study  of  the  behavior  of 
cut  sprigs  of  different  species  of  plants  when  subjected  to  laboratory 
air  and  to  illuminating  gas.  He  notes  the  fact  that  under  either  of 
the  above  conditions  all  the  flowers  and  occasionally  a  few  small 
shoots  are  abscissed.  He  finds,  however,  that  not  all  the  species  in  a 
given  family  behave  similarly  in  response  to  these  conditions.  We 
are  particularly  interested  in  the  Solanaceae  and  we  may  note  that 
this  family  contained  more  species  that  detached  their  flowers  in 
illuminating  gas  than  any  other  of  the  families  investigated  by  Han- 
nig. According  to  Fitting  (1911),  narcotic  vapors  such  as  tobacco 
smoke,  carbon  dioxide,  ether,  chloroform  or  illuminating  gas  fre- 
quently cause  premature  abscission  of  the  corolla.  He  notices,  how- 
ever, that  ammonia  or  turpentine  vapors  fail  to  cause  abscission. 
Brown  and  Escomb  (1902)  make  the  statement  that  Nicotiana,  Cu- 
curbita,  and  Fuchsia  shed  flowers  and  buds  in  air  containing  only 
0.114  per  cent  carbon  dioxide. 


360  University  of  California  Publications  in  Botany         [V<>L- 


fc.  INDUCTION  BY  MECHANICAL  INJUEY 

Becquerel  (1907),  in  a  brief  paper  on  the  effect  of  wounding 
flowers  of  Nicotiana,  notes  that  even  after  fifteen  days  flowers  without 
sepals,  anthers,  or  stigmas  do  not  fall.  After  the  same  length  of  time, 
flowers  without  corollas  or  flowers  in  which  the  corolla  or  stamens  are 
only  half  removed,  have  fallen.  He  points  out  that  this  result  is  more 
conspicuous  in  young  flowers  but  did  not  investigate  this  point  suffi- 
ciently to  arrive  at  any  definite  conclusions.  According  to  Hannig, 
removal  of  various  organs  of  flowers  frequently  causes  abscission  but 
wounding  of  the  pedicel  does  not.  He  concludes,  therefore,  that  in- 
jury itself  does  not  cause  abscission  but  only  acts  indirectly  by  inter- 
fering with  important  physiological  processes  in  the  treated  tissues. 

According  to  Lloyd  (1914a),  shedding  of  very  young  cotton-bolls 
can  be  induced  by  removal  of  the  styles  before  pollination,  but  fall  in 
this  case  can  be  assigned,  as  Fitting  has  shown,  to  lack  of  fertilization. 
It  appears  that  in  the  cotton  flower  (Lloyd,  1916&)  there  is  an  inhibi- 
tion period  which  starts  with  the  opening  of  the  corolla  and  during 
which  premature  abscission  as  the  result  of  sudden  stimuli  very  sel- 
dom occurs.  Also,  cotton-bolls  larger  than  30  mm.  in  diameter  are 
very  seldom  shed  under  any  conditions.  Other  results  obtained  by 
Lloyd  on  the  effect  of  injury  on  the  abscission  of  cotton  flowers  are 
discussed  above  under  "Time  of  Abscission"  (page  357).  Lloyd 
(1914&)  also  notes  the  effect  of  injury  on  abscission  of  internodes  in 
Impatiens  Sultani.  Plants  of  this  species,  when  a  cut  is  made  across 
the  stem,  cast  off  the  remainder  of  the  severed  internode.  He  gives 
results  of  experiments  on  the  effect  of  different  types  of  injury,  noting 
that  some  severe  injuries  do  not  cause  abscission.  Gortner  and  Harris 
(1914)  have  obtained  similar  results  with  the  same  species.  They 
find  that  when  the  cut  is  made  across  the  internode,  very  close  to 
the  separation  layer,  abscission  usually  occurs,  but  occasionally  it  does 
not.  They  state,  as  does  Lloyd,  that  the  shape  and  location  of  the 
separation  layer  may  vary  slightly  according  to  the  type  of  injury. 

c.  THE  DIEECT  OE  INDIEECT  ACTION  OF  THE  EXTEENAL 
STIMULUS 

In  all  the  above  investigations  the  question  naturally  arises, 
whether  the  narcotic  vapors  and  injuries  or  any  stimulus  conducive 
to  abscission  act  indirectly  through  their  influence  on  the  physiolog- 
ical condition  of  the  plant  or  directly,  through  their  action  on  the 
cells  of  the  separation  zone.  Most  investigators,  except  Wiesner,  ex- 


19181    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    361 

press  the  opinion  that  atmospheric  factors  work  directly  in  causing 
"spontaneous"  abscission,  although  offering,  so  far  as  I  can  see,  no 
evidence  for  this  view.  Fitting  states  that  the  external  influence 
acts  directly  in  most  cases,  but  that  the  indirect  action  is  apparent  in 
forms  which  must  build  a  separation  layer  before  fall  can  occur. 
In  regard  to  the  action  of  injury,  it  seems  to  be  the  opinion  of  most 
investigators  (Hannig,  Bacquerel,  Gortner  and  Harris)-- -that  the 
stimulus  acts  indirectly  by  interfering  in  some  way  with  such 
important  physiological  processes  as  transpiration,  respiration,  or 
assimilation.  On  the  other  hand,  if  abscission  is  sometimes  a  semi- 
tropistic  phenomenon,  as  Fitting  has  suggested,  it  is  evident  that 
injury  may  act  directly  in  causing  flower-fall. 

TECHNIQUE 

The  results  noted  below  were  obtained  largely  from  the  examina- 
tion of  microscopic  preparations  made  by  the  paraffin  method, 
although  this  method  was  supplemented  by  free-hand  sections  mounted 
in  water.  In  investigating  the  condition  of  the  pedicel  in  some  species 
(Datura  sp.,  Petunia  sp.  and  several  species  of  Nicotiana)  only  free- 
hand sections  were  examined.  For  most  microchemical  studies  fairly 
thick,  free-hand  sections  are  preferable.  The  material  for  sectioning 
in  paraffin  was  killed  and  fixed  in  various  concentrations  of  the 
chromo-acetic  series  and  dehydration  and  infiltration  were,  in  general, 
carried  on  very  slowly.  The  free-hand  sections  were  mounted  in  water 
without  killing. 

In  cutting  longitudinal  sections  of  any  kind  all  the  pedicels  were 
oriented  so  that  the  sections  were  cut  parallel  to  the  main  stem  of  the 
inflorescence,  in  the  plane  formed  by  the  pedicel  and  stem  taken 
together.  In  studying  the-  histology  of  the  pedicel  and  the  cytology 
of  the  separation  layer  and  in  studjdng  the  method  of  cell  separation, 
these  longitudinal  sections  were  supplemented  by  cross  sections  in 
series  through  the  base  of  the  pedicel.  It  was  impossible  to  cut  very 
thin,  longitudinal  sections  in  paraffin  without  crushing  or  breaking 
the  cells ;  most  of  these  sections  therefore  were  cut  from  10/x  to  15/*  in 
thickness.  For  a  similar  reason,  it  was  found  necessary  to  cut  thick 
sections  (20/x  to  25/*)  of  the  pedicels  of  fruits  in  which  mechanical  tissue 
had  developed.  It  was  possible,  however,  to  cut  excellent  paraffin 
sections  from  5/x,  to  1^  in  thickness  in  cross-section  or  longitudinally 
through  the  small  cells  of  the  separation  zone.  Since  the  cells  of  the 


362  University  of  California  Publications  in  Botany         [VOL.  5 

separation  zone  are  very  small,  not  much  could  be  determined  in 
regard  to  the  dissolution  of  cell  walls  by  means  of  thick,  free-hand 
sections.  The  best  results  along  this  line  were  obtained  from  the  thin 
paraffin  sections  of  the  separation  zone,  although  in  order  to  show  the 
cell  wall  in  its  normal  thickness  it  was  necessary  to  use  the  free-hand 
sections.  As  a  supplement  to  these  sections,  several  points  of  interest 
were  brought  out  by  washing  off  the  isolated  cells  from  the  end  of 
freshly  abscissed  pedicels  and  mounting  them  for  microscopic  exam- 
in.ation. 

In  most  of  the  work  the  paraffin  sections  were  stained  in  safraniii 
and  Delafield's  haematoxylin.  The  free-hand  sections  were  generally 
mounted  in  water  and  stained  in  iodine.  In  special  instances  other 
stains  were  used.  Thus,  in  testing  for  chemical  differences  in  the  cell 
walls  of  the  separation  cells,  several  other  stains,  such  as  erythrosin, 
eosin,  Bismark  brown,  gentian  violet  and  Ruthenium  red  were  used. 
It  was  found  that  for  demonstrating  the  dissolution  of  cell  walls 
aqueous  methylene  blue  was  an  excellent  stain  to  use.  This  stain  was 
allowed  to  act  overnight  and  the  sections  destained  slightly  in  alcohol. 
Methylene  blue  was  also  an  excellent  stain  for  the  isolated  cells  ob- 
tained as  noted  above.  By  fixing  these  cells  to  the  slide  with  albumen 
fixative  and  staining  with  this  stain,  the  thin  membranous  wall  sur- 
rounding the  protoplast  can  be  distinctly  seen. 

Various  methods,  such  as  subjecting  inflorescences  to  illuminating 
gas  and  mechanical  injury,  were  used  to  bring  about  abscission.  The 
best  results  were  obtained  in  cases  where  abscission  was  induced  by 
inserting  shoots  under  a  bell-jar  containing  from  1.5  per  cent  to 
3  per  cent  illuminating  gas.  By  using  illuminating  gas  in  this  way 
and  by  taking  sections  of  the  pedicels  at  intervals  it  was  possible  to 
determine  just  when  the  first  signs  of  abscission  appeared  in  a  certain 
percentage  of  gas.  This  time  was  definitely  determined  for  certain 
species  so  that  it  was  possible  to  get  material  killed  and  fixed  at  any 
desired  stage  in  the  process  of  abscission.  It  was  found  that  the  best 
results  were  obtained  by  killing  and  fixing  the  pedicels  at  about  the 
time  when  abscission  was  known  to  be  commencing. 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    363 


HISTOLOGY  AND   CYTOLOGY   OF   THE   PEDICEL 

1.    HlSTOLOGICAL    AND    CYTOLOGICAL    CONDITIONS    OF    THE 

MATURE  PEDICEL 
a.  NICOTIANA 

The  vascular  system  in  Nicotiana,  as  in  all  the  other  genera 
examined,  is  characterized  by  intraxylary  phloem.  Nicotiana  differs 
slightly  from  all  others  in  that  the  xylem  seems  in  cross-section 
to  be  composed  of  a  continuous  ring  of  radial  strands  of  tracheae 
rather  than  composed  of  a  broken  ring  of  distinct  bundles.  When  a 
branch  of  the  vascular  system  (fig.  1,  a)  containing  twenty  to  thirty 
xylem  strands  is  given  off  to  the  pedicel,  it  assumes  the  shape  of  a 
crescent  in  cross-section,  with  the  opening  of  the  crescent  on  the  ven- 
tral side.  A  short  distance  distal  to  the  groove  which  marks  the  sep- 
aration zone  (fig.  1,6),  the  crescent  closes  and  throughout  the 
remainder  of  the  pedicel  the  vascular  system  forms  a  complete  cylin- 


c - 


Fig.  1.    Diagram  of  pedicel  of  Nicotiana 


a — vascular  system. 
b — separation    zone. 
c — pedicel  cortex. 
sc — stem  cortex. 
e — epidermis. 


f — chlorophyllous  tissue. 
g — groove. 
h — separation  layer. 
p — pedicel  pith. 


364  University  of  California  Publications  in  Botany         [VOL.  5 

der.  The  pith  and  cortex  (fig.  1,  p  and  c)  are  composed  of  large 
parenchyma  cells  which  in  the  cortex  are  two  or  three  times  as  long 
as  wide,  but  in  the  pith  are  more  nearly  isodiametric.  There  is  no 
mechanical  tissue  to  be  found  in  the  floral  pedicel  but,  as  will  be  noted 
in  more  detail  later,  wood  fibres  are  formed  as  soon  as  the  fruit  begins 
to  develop.  The  epidermis  of  the  pedicel  (fig.  1,  e)  is  typical  but  with 
a  poorly  developed  cuticle,  especially  in  the  groove  (fig.  1,  #),  where 
the  cells  are  also  much  reduced  longitudinally.  Beneath  the  epidermis 
is  a  layer  of  small  cells  with  very  large  intercellular  spaces  and  an 
abundance  of  chloroplasts  (fig.  I,/).  This  tissue  stops  a  short  dis- 
tance proximal  to  the  separation  zone  and  does  not  continue  in  the 
pedicel.  The  layer  of  collenchyma  which  is  commonly  found  in  cer- 
tain species  just  beneath  this  chlorophyl  tissue  is  entirely  absent  in 
Nicotiana,  or  at  least  is  very  poorly  developed. 

Corresponding  with  the  general  region  of  the  groove  is  an  area  of 
medullary  and  cortical  cells  which  are  smaller  than  corresponding 
cells  on  either  the  proximal  or  distal  side  of  the  groove.  This  region 
of  small  cells  is  homologous  with  the  separation  zone  (fig.  1,  &)  and  it 
extends  across  the  base  of  the  pedicel.  The  smallest  cells  are  in  the 
center  of  the  region,  in  a  plane  with  the  bottom  of  the  groove,  and 
grade  in  size  to  the  larger  cells  of  the  pith  and  cortex  on  either  side 
(plate  49,  fig.  1).  The  zone  of  small  cells  is  ten  to  fifteen  tiers  of 
cells  thick  on  the  dorsal  side  but  is  wider  on  the  ventral  side,  where  it 
spreads  out  into  the  large  area  of  storage  cells  found  in  the  axil  of 
the  pedicel.  The  separation  layer  (fig.  1,  h)  is  located  five  to  seven  tiers 
of  cells  distal  from  the  bottom  of  the  groove.  Hanning  reports  this 
layer  as  occurring  at  the  tip  of  the  pedicel  in  Nicotiana  Langsdorffii, 
but  in  all  my  experiments  on  two  varieties  of  this  species  I  find  separa- 
tion invariably  occurring  at  the  base  of  the  pedicel  in  the  position 
described  above.  All  the  species  and  varieties  of  Nicotiana  examined 
show  a  structure  of  the  pedicel  corresponding  with  the  above  descrip- 
tion except  that  in  some  varieties,  as  in  those  of  N.  Bigelovii,  the  sep- 
aration zone  is  much  thinner  on  the  dorsal  side.  In  such  cases  it  is 
also  noted  that  the  groove  is  poorly  developed. 

The  cells  of  the  separation  layer  are  in  no  way  morphologically 
differentiated  from  those  making  up  the  remainder  of  the  separation 
zone.  Indeed,  any  cell  of  the  zone  seems  capable  of  functioning  as  a 
separation  cell.  The  separation  cells  are  smaller  than  normal  cortical 
cells  and  spherical  in  shape  except  in  the  vascular  bundles,  where  they 
do  not  seem  to.be  differentiated  in  size  and  are  elongated  parallel  to 
the  longitudinal  axis  of  the  pedicel.  The  cell  walls  are  slightly  thicker 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanac&ae    365 


than  the  walls  of  normal  cortical  cells,  especially  at  the  corners,  thus 
giving  the  tissue  a  somewhat  collenchymatous  appearance.  The  small- 
est cells  more  proximal  show  this  collenchymatous  nature  more  strik- 
ingly than  do  the  others.  No  difference  in  chemical  composition  could 
be  detected,  by  means  of  microchemical  tests  using  caustic  potash, 
sulfuric  acid,  nitric  acid,  and  various  stains,  between  the_cell  walls  of 
the  separation  cells  and  walls  of  other  cortical  cells.  Other  tests,  how- 
ever, indicated  a  difference  in  the  nature  of  the  cell  contents  in  the 
two  types  of  cells.  Iodine  frequently  indicates  the  presence  of  starch 
in  these  cells  and  also  colors  the  protoplasts  a  darker  brown  than  in 
normal  cells,  showing  that  the  separation  cells  are  rich  in  protoplasm. 
The  amount  of  starch  in  the  cells,  however,  was  found  to  be  extremely 
variable,  ranging  from  a  total  absence  of  starcli  to  an  abundance  of 
it.  Iodine  green  imparts  to  the  protoplast  of  the  separation  cells  a 
deep  blue  color  in  contrast  with  other  cortical  cells,  which  are  not 
colored  by  this  stain.  The  blue  reaction  is  most  prominent  where  the 
separation  layer  crosses  the  phloem.  Other  cells  which  react  in  the 
same  way  to  this  stain  are  the  sieve  tubes  and  companion  cells  and 
the  storage  cells  in  the  axil  of  the  pedicel. 

b.  LYCOPERSICUM 

Conditions  in  Lycopersicum  differ  in  certain  respects  from  those 
existing  in  Nicotians.  In  the  former  the  separation  zone  (fig.  2,  a) 
seems  to  be  located  at  the  middle  of  the  pedicel 
and  is  marked  externally  by  a  swelling,  as  well 
as  by  the  groove  of  the  type  already  noted  as 
characteristic  of  the  pedicel  of  Nicotiana.  This 
groove  in  the  tomato  is  very  deep  (plate  53, 
fig.  1),  reaching  fully  half  the  depth  of  the 
cortex,  and  is,  furthermore,  of  about  the  same 
depth  all  the  way  round,  differing  in  this 
respect  from  Nicotiana,  where  the  groove  is 
absent  or  poorly  developed  on  the  ventral 
side.  The  vascular  system  in  Lycopersicum 
(fig.  2,  &),  in  contrast  with  the  condition  in 
Nicotiana,  is  composed  of  scattered  bundles 
of  xylem  which  in  this  case  do  not  form  a 
crescent  proximal  to  the  groove  but  are  in  the 
form  of  a  complete  cylinder  throughout  the 
entire  pedicel.  Beneath  the  epidermis  (fig. 


r ~C 

Fig.  2.    Diagram  of  pedicel 

of  Lycopersicum 
a — separation  zone, 
ft — vascular  system. 
c — epidermis. 
<i — separation  layer. 
e — pith. 
f — chlorophyl-bearing 

tissue. 
g — collenchyma 


366  University  of  California  Publications  in  Botany         [VOL.  5 

2,  c)  is  the  chlorophyl-bearing  region  of  the  cortex  (fig.  2,  f),  such 
as  occurs  in  Nicotiana,  but  in  this  case  the  tissue  continues  in  the 
pedicel  distal  to  the  groove.  Beneath  this  chlorophyl-bearing  tissue 
is  a  layer  of  well-developed  collenchyma  (fig.  2,  g)  which  however 
does  not  continue  in  the  pedicel  distal  to  the  groove.  The  separation 
layer  (fig.  2,  d)  consists  of  three  to  six  tiers  of  cells  and  is  located 
in  a  plane  with  the  groove,  differing  in  this  respect  from  Nicotiana, 
where  it  is  located  a  short  distance  distal  to  the  groove.  Correspond- 
ing to  the  condition  in  Nicotiana,  the  chief  characteristic  of  the  separa  - 
tion  cells  is  their  small  size,  spherical  outline  and  active  physiological 
condition. 

o.  OTHEE  GENERA  OF  THE  SOLANACEAE 

The  condition  of  the  pedicel,  so  far  as  the  histology  of  the  separa- 
tion zone  is  concerned,  was  examined  in  several  other  species,  a  list 
of  which  is  given  below : 

Solanum  jasminioides  Oestrum  fasciculatum 

Solanum  tuberosum  lochroma  tuberosa 

Solanum  verbascifolium  Datura  sanguineum 

Solanum  umbelliferum  Salpichrora  rhomboidea 

Solanum  nigrum  Petunia  hybrida 

Solanum  marginatum  Salpiglossis  sinuata 

Lycium  australis 

The  general  condition  of  the  pedicel  of  Datura  sanguineum  and 
Petunia  hybrida  is  worth  describing  in  some  detail.  The  tissues  of 
plants  of  D.  sanguineum  are  more  or  less  herbaceous  in  nature,  large- 
celled  and  somewhat  succulent  throughout.  The  chlorophyl-bearing 
tissue  which,  in  striking  contrast  with  the  condition  in  Nicotiana  and 
Lycopersicum  (figs.  1  and  2),  is  continuous  over  the  separation  zone, 
is  composed  of  two  rows  of  small,  spherical  cells  just  beneath  the 
epidermis.  Except  for  a  layer  of  collenchyma,  whose  much  elongated 
cells  extend  the  entire  length  of  the  pedicel  and  thus  continue  the  col- 
lenchyma through  the  separation  layer,  the  cortex  and  pith  are  com- 
posed of  more  or  less  isodiametric,  thin-walled  cells.  Floral  abscission 
is  as  common  in  this  species  as  it  is  in  Nicotiana.  The  flowers  are 
very  large  and  furnish  excellent  material  for  a  study  of  the  cytology 
of  abscission.  Unfortunately  not  a  sufficient  number  of  flowers  could 
be  obtained  to  make  possible  any  detailed  study  of  this  genus.  It  was 
noticed,  however,  that  there  is  no  region  of  small  cells  at  the  base  of 
the  pedicel  within  which  separation  occurs  and  that  the  separation 
cells  are  identical  in  size  and  shape  with  those  on  either  side  among 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    367 

which  separation  does  not  occur.  The  separation  layer  here  is  located 
about  8  mm.  distal  to  the  base  of  the  pedicel,  with  absolutely  no  ex- 
ternal indication  of  its  position.  Microchemical  tests,  which  in  Nioo- 
tiana  gave  different  reactions  in  the  case  of  the  separation  zone  and  in 
the  case  of  normal  cortical  cells,  here  fail  to  show  any  corresponding 
condition  of  differentiation. 

Abscission  has  never  been  found  to  occur  in  Petunia  or  Salpiglossis, 
so  that  it  is  of  interest  to  examine  the  histological  condition  of  the 
base  of  the  pedicel  in  these  two  species.  They  are  practically  identical 
with  regard  to  the  structure  of  the  pedicel,  so  that  the  description 
given  below  can  be  taken  as  applying  to  both  genera.  The  cortical 
cells  of  the  pedicel  pass  into  those  of  the  stem  without  any  groove  or 
small-celled  region.  On  the  ventral  side,  however,  is  the  region  of 
small  cells  in  the  axis  of  the  pedicel,  which  is  more  or  less  common  to 
all  flowers.  The  tissues  of  Petunia  are  not  so  soft  and  succulent  as 
those  of  Datura,  Nicotiana,  and  Lycopersicum.  They  tend  rather  to 
be  dry  and  tough.  The  cells  in  the  cortex  and  pith  are  also  not  so 
nearly  isodiametric  as  in  Datura,  but  are  much  elongated  in  a  direc- 
tion parallel  with  the  long  axis  of  the  pedicel. 

The  condition  in  the  other  species  mentioned  above  will  be  given 
only  a  general  description.  Abscission  occurs  in  all  the  other  species 
except  Salpichrora  and  Lycium  which,  however,  do  not  differ,  in 
respect  to  the  histology  of  the  base  of  the  pedicel,  from  any  of  the 
others.  Solanum  tuberosum  resembles  Lycopersicum.  All  the  other 
species  are  similar  in  regard  to  the  structure  of  the  separation  zone. 
There  is  in  every  case  a  general  region  of  small  cells  extending 
across  the  base  of  the  pedicel  where  the  separation  layer  occurs. 

3.  DEVELOPMENT  OF  THE  SEPARATION  ZONE  IN  Lycopersicum 
AND  Nicotiana 

a.  LYCOPEESICUM 

The  development  of  the  separation  zone  could  be  followed  better 
in  Lycopersicum  than  in  Nicotiana  because  in  the  former  the  zone  is 
not  so  close  to  the  main  axis  of  inflorescence.  The  problem  here 
resolves  itself  into  an  effort  to  determine,  by  means  of  longitudinal 
sections  of  very  young  pedicels,  how  early  in  the  development  of  the 
flower  the  groove  and  the  differentiation  in  cell  size  of  the  separation 
cells  appear.  It  was  found  that  the  development  of  the  separation  zone 
indicates  the  method  by  which  the  groove  and  differentiation  in  cell 


368 


University  of  California  Publications  in  Botany         [VOL.  5 


size  originate.  The  groove  is  fairly  well  developed  (fig.  5)  in  young 
buds  whose  corolla  is  only  3  mm.  in  length,  but  is  not  so  deep  as  in 
older  buds.  The  cells  of  the  separation  zone  at  this  stage  are  smaller 
than  cells  on  either  side,  but  the  difference  is  not  so  prominent  as  in 
older  flowers.  In  very  small  buds  whose  corolla  is  only  1  mm.  in 
length  or  whose  calyx  is  only  2  mm.  long,  the  groove  is  just  beginning 
to  appear  (fig.  4).  In  buds  below  this  size  (fig.  3)  no  groove  or 
differentiation  in  cell  size  can  be  detected.  Abscission  can  occur  in 
these  early  stages,  before  the  groove  or  differentiation  in  the  size  of  the 
separation  cells  has  appeared,  as  well  as  at  any  other  stage.  In  these 
early  stages  the  radial  diameter  of  the  cortex  is  much  less,  as  com- 
pared with  that  of  the  pith,  than  in  older  flowers.  It  is  evident, 
therefore,  that  the  cells  of  the  separation  zone  are  small  because  they 
retain  their  original  small  size  while  the  rest  of  the  cortical  cells 
increase  in  size.  The  fact  that  the  groove  is  formed  makes  it  probable 
that  there  have  been  few  cell  division,  or  none,  in  the  separation  zone 
of  the  cortex  during  the  development  of  the  bud.  It  was  observed, 
however,  that  the  cells  of  the  separation  zone  in  the  pith  retain  their 
meristematic  nature  for  a  considerable  period  during  the  development 


r 


/ 


Fig.  3 


Fig.  4 


Fig.  5 
a — separation  zone 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    369 

of  the  flower.  They  are  at  this  time  rectangular  in  shape,  elongated 
perpendicularly  to  the  long  diameter  of  the  pedicel,  and  arranged  in 
longitudinal  rows.  In  later  stages,  however,  wrhen  the  flower  is  at 
anthesis,  or  the  fruit  is  forming,  these  cells  have  rounded  up  and 
become  irregularly  arranged,  thus  leaving  rather  large  intercellular 
spaces. 

fc.  NICOTIANA 

The  separation  zone  develops  in  Nicotiana  much  as  it  does  in 
Ly coper sicum.  It  was  observed  in  very  young  buds — calyx  2  or  3  mm. 
or  shorter — that  no  groove  was  present.  In  buds  larger  than  these, 
the  groove  and  small  size  of  the  separation  cell  is  apparent,  appearing 
first  on  the  dorsal  side  of  the  pedicel.  It  is  evident  that  in  both  these 
genera  the  groove  and  area  of  small  cells  are  explained  in  the  same 
way,  i.e.,  by  the  fact  that  the  normal  cortical  cells  increase  in  size 
faster  than  do  the  cells  of  the  separation  zone.  Since  in  both  genera 
abscission  can  occur  even  before  differentiation  of  any  kind  appears  at 
the  base  of  the  pedicel,  it  is  evident  that  the  groove  and  small-celled 
region  do  not  necessarily  bear  any  relation  to  abscission.  This  state- 
ment is  borne  out  by  the  fact  that  in  Datura  neither  the  groove  nor 
the  area  of  small  cells  is  present  and  in  Nicotiana  separation  occurs  a 
short  distance  distal  to  the  groove. 

c.  CONCLUSIONS  FBOM  THE  STUDY  OF  THE  DEVELOPMENT  OF 
THE  SEPAEATION  ZONE 

In  view  of  the  above  discussion  it  is  clear  that  the  separation  layer 
in  Ly  coper  sicum,  Nicotiana,  Datura,  and  probably  in  the  other  genera 
noted,  originates  according  to  the  first  method,  a,  proposed  by  Kubart 
(cf.  page  350).  That  is  to  say,  the  separation  layer  represents  merely 
a  portion  of  the  primary  meristem  which  retains  its  original  physi- 
ological capacities. 

4.  INCREASE  IN  SIZE  AND  DEVELOPMENT  OF  MECHANICAL  TISSUES  IN 
THE  PEDICEL  OF  Nicotiana  AND  Lycopersicum 

There  is  a  marked  increase  in  the  size  of  the  pedicel  in  both 
Nicotiana  and  Lycopersicum  during  the  development  of  the  fruit.  It 
was  found  that  during  this  development  the  diameter  of  the  pith 
remains  about  the  same,  the  actual  increase  in  size  being  almost 
entirely  confined  to  the  cortex  (cf.  figs,  3,  4,  and  5).  This  increase  in 
the  diameter  of  the  cortex  in  the  pedicel  of  Nicotiana  is  due,  in  the 
first  place,  to  an  increase  in  the  size  of  the  original  cortical  cells, 


370  University  of  California  Publications  in  Botany         [VOL.  5 

which  in  average  cases  measured  about  20/x  in  diameter  in  the  flower 
and  about  40/x,  in  the  fruit.  In  the  second  place,  it  is  due  to  four  or 
five  divisions  of  the  cambium  layer.  This  second  factor  in  the  increase 
in  size  of  the  pedicel  becomes  evident  when  a  count  is  made  of  the  cells 
between  the  phloem  and  tracheae,  the  result  giving  approximately  six 
cells  in  the  flower  and  eleven  in  the  fruit. 

The  increase  in  size  of  the  pedicel  of  Lycopersicum,  which  is  much 
more  prominent  than  the  increase  in  Nicotiana,  can  be  explained  in 
the  same  manner.  In  the  former  the  increase  in  size,  which  in  this 
case  takes  place  almost  entirely  distal  to  the  groove,  may  proceed  to 
such  an  extent  that  the  diameter  of  the  pedicel  of  the  fruit  is  two  or 
three  times  that  of  the  flower  at  anthesis.  A  measurement  of  the 
cortical  cells  in  cross-section  gave  on  the  average  10//,  in  the  flower  and 
28/x  in  the  fruit.  In  this  case  only  two  or  three  divisions  of  the 
cambium  occur;  the  cells  resulting  immediately  show  lignification. 

The  next  subject  of  consideration  is  the  development  of  mechanical 
tissue  in  the  pedicel  of  Nicotiana  and  its  relation  to  abscission.  It  will 
be  remembered  that  there  was  no  mechanical  tissue  noted  in  the 
pedicels  of  buds  and  flowers.  Parallel  with  the  development  of  the 
fruit,  however,  a  continuous  ring  of  mechanical  tissue  appears  in  the 
xylem  of  the  pedicel.  This  mechanical  tissue  is  evidently  the  result 
of  a  gradual  lignification  of  the  cells  of  the  cambium  and  the  outside 
portion  of  the  xylem  parenchyma.  There  is  thus  formed  a  continuous 
sheath  of  what  may  best  be  called  wood-fibre  tissue,  in  the  form  of  a 
cylinder  just  outside  the  tracheal  elements.  These  mechanical  elements 
first  appear  in  the  tissues  of  the  pedicel  five  or  six  days  after  anthesis, 
but  since  the  lignification  in  these  more  distal  tissues  is  merely  the 
result  of  the  spreading  upwards  of  the  lignification  in  the  older  parts 
of  the  plant,  this  period  depends  somewhat  on  the  position  of  the 
flower  on  the  inflorescence.  It  was  noticed  in  Nicotiana  that  the 
wood-fibre  tissue  develops  on  both  sides  of  the  separation  zone  before 
appearing  in  the  latter,  but  in  time  it  becomes  continuous  through 
the  separation  layer.  By  a  lignification  of  the  cells  between  the  two 
points  of  the  crescent  of  wood  in  the  separation  zone,  there  is  also 
a  slight  tendency  to  close  this  crescent  on  the  ventral  side. 

Since  abscission  has  not  been  observed  to  occur  in  lignified  cells, 
the  question  at  once  arises  whether  the  tough  sheath  of  lignified  cells 
which  continues  through  the  separation  layer  could  hold  the  fruit  on 
the  stem  even  after  actual  abscission  had  occurred.  Upon  looking 
over  any  large  number  of  plants  in  the  field  it  will  at  once  be  evident 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    371 

that  such  a  condition  of  affairs  very  often  exists.  It  will  be  found  in 
many  cases,  especially  on  older  plants,  that  although  abscission  has 
occurred  in  the  cortex,  as  evidenced  by  the  presence  af  a  white, 
powdery  substance  at  the  base  of  the  pedicel,  the  capsule  is  yet  firm 
on  the  stem.  Indeed,  in  certain  hybrid  tobaccos  it  is  common  to  find 
most  of  the  capsules  in  this  abscissed  condition.  The  fruit  is  supported 
in  these  cases  by  the  tough  mechanical  elements  of  the  wood,  which 
also  prevent  the  breaking  of  the  tracheae  and  protect  the  intraxylary 
phloem.  In  the  pith  the  tissues  may  be  in  a  somewhat  abscissed  con- 
dition, but  since  there  is  no  way  for  these  cells  to  escape  through  the 
sheath  of  wood  they  remain  for  some  time  in  position  before  finally 
collapsing. 

The  development  of  mechanical  tissues  takes  place  in  Lycopersicum 
in  much  the  same  manner  as  in  Nicotiana  but  with  the  distinct  differ- 
ence that  in  the  former  the  wood-fibre  tissue  does  not  become  con- 
tinuous through  the  separation  layer.  That  is  to  say,  in  the  tomato 
a  break  is  .left  in  the  mechanical  tissue  in  a  plane  with  the  bottom  of 
the  groove.  It  is  evident  here  that  abscission  would  cause  fall  of  the 
fruit  in  any  stage  of  its  development,  although  in  this  case  it  happens 
that  abscission  very  rarely  occurs  after  two  or  three  days  past  anthesis. 
A  condition  resembling  this  one  in  the  tomato  was  observed  in  other 
berry-forming  species  of  the  Solanaceae  such  as  Oestrum  fasciculatum 
and  Solanum  verbascifolium,  which  often  drop  their  immature  fruits 
by  abscission.  Abscission,  however,  very  seldom  occurs  in  mature 
berries  of  these  species,  the  fruit  generally  falling  away  from  the 
receptacle  above  the  calyx. 


THE  PROCESS   OF  ABSCISSION 

1.  GENERAL  DESCRIPTION  OP  THE  PROCESS  IN  SEVERAL  GENERA 

a.  NICOTIANA 

The  process  of  abscission  in  all  the  species  of  Nicotiana  investi- 
gated conforms  to  the  usual  type  involving  separation  and  isolation  of 
cells.  Further  details  of  the  process  were  briefly  discussed  in  a  pre- 
liminary paper  (Goodspeed  and  Kendall,  1916)  for  certain  F:  species 
hybrids  of  Nicotiana.  It  was  there  noted  that  cell  separation  starts  in 
the  dorsal  side  of  the  pedicel,  in  the  cortex  a  short  distance  distal  to 
the  groove  (pi.  49,  fig.  1)  and  spreads  from  this  point  around  to  the 
ventral  side.  The  first  external  indication  seems  to  be  a  bulging  of 


372  University  of  California  Publications  in  Botany         [VOL.  5 

the  epidermis  (pi.  49,  fig.  2)  over  the  tissue  in  which  the  process  is 
taking  place.  Simultaneously  with  the  start  of  abscission  in  the 
cortex,  the  process  apparently  originates  independently  in  the  pith 
(pi.  50,  fig.  1).  It  was  further  noted  that  the  number  of  cells  con- 
cerned in  the  process,  as  a  general  rule,  is  greater  in  the  hybrids  than 
in  their  parents  and  also  that  this  is  true  of  ' '  automatic ' '  as  compared 
with  ''spontaneous"  abscission.  Just  beneath  the  epidermis  the  cells 
involved  in  separation  were  reported  as  being  from  five  to  ten  tiers 
thick,  but  as  the  process  approached  the  vascular  tissue  the  separation 
layer  was  evidently  reduced  in  thickness  to  not  over  one  or  two  tiers 
of  cells  (pi.  52,  fig.  1).  In  the  pith  a  more  or  less  spherical  mass  of 
cells  is  involved  (pi.  50,  fig.  1).  When  the  separation  is  completed 
the  flower  may  remain  in  position  for  some  time,  until  the  epidermis 
and  tracheal  elements  are  broken  by  some  mechanical  agency. 

The  exposed  separation  surface  of  the  pedicel  was  stated  to  be 
convex  in  outline  and  slightly  notched  at  the  tip.  Upon  closer  exam- 
ination the  surface  itself  was  seen  to  be  composed  of  the  protruding, 
rounded  ends  of  cells  with  here  and  there  completely  isolated  cells  and 
broken  ends  of  spiral  tracheae.  These  isolated  cells  are  apparently 
normal  and  do  not  markedly  differ  in  form,  size,  or  in  the  nature  of 
their  cell  inclusions  from  the  same  cells  before  separation.  The 
exposed  surface  of  the  attached  portion  of  the  pedicel  is  similar  in 
appearance  to  that  of  the  detached  portion,  but  is  more  or  less  flat 
in  outline.  After  separation  the  cells  on  this  surface  collapse  and 
probably  act  as  a  protective  layer. 

Following  the  observations  recorded  above,  which  had  to  do  largely 
with  flower-fall  in  the  F^  species  hybrids,  a  number  of  species  have 
been  investigated  in  an  effort  to  determine  whether  or  not  their  mode 
of  abscission  differs  from  that  already  described. 

It  may  be  noted  at  the  start  that  no  marked  exceptions  were  found 
to  the  previously  described  condition,  although  at  least  two  stages  in 
the  process  of  abscission  have  been  found  to  be  subject  to  considerable 
variation.  The  first  of  these  stages  has  to  do  with  the  place  of  origin 
of  the  abscission  process  itself.  An  independent  origin  in  the  pith 
has  been  demonstrated  to  occur  in  a  large  number  of  species  and 
occasionally  it  was  found  that  the  first  evidences  of  abscission  could 
be  detected  here  before  any  similar  evidences  appeared  in  the  cortex. 
Again,  it  was  found  in  most  species  that  cell  separation  starts  first  in 
the  ventral  cortex  although  other  places  of  origin  were  found  in 
several  cases.  Thus,  in  Nicotiana  Tabacum  "Maryland"  and  F,  TIM. 


Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    373 

for  example,  the  process  originates  on  the  ventral  side  and  may  even 
spread  through  the  large  area  of  storage  cells  in  the  axil  of  the  pedicel 
before  reaching  the  dorsal  side.  The  distance  distal  from  the  bottom 
of  the  groove  at  which  separation  appears  is  also  subject  to  variation. 
This  variation,  however,  is  not  typical  of  certain  species,  since  it  may 
occur  at  different  times  in  the  same  species,  evidently  as  a  result  of 
an  abnormal  stimulation  to  abscission. 

The  second  part  of  the  process  subject  to  variation  has  to  do  with 
the  amount  of  tissue  that  may  be  concerned  in  actual  cell  separation. 
Abscission  first  becomes  complete  in  a  narrow  plane  between  two  or 
three  tiers  of  cells  across  the  pedicel  and  the  flower  can  be  easily 
shaken  off  at  that  time.  If,  however,  the  flower  remains  on  the  stem, 
and  is  kept  turgid  by  the  water  rising  in  the  unbroken  trachea?,  cell 
separation  spreads  more  and  more  widely  through  the  tissues  of  the 
pedicel,  especially  in  the  pith  and  cortex.  It  is  the  extent  to  which  this 
spreading  normally  proceeds  that  varies  in  the  different  species.  When 
the  process  has  spread  to  a  considerable  extent,  a  white  ring  formed 
by  the  isolated  masses  of  cells  can  be  seen  with  the  naked  eye  at  the 
base  of  the  pedicel  and  a  casual  inspection  indicates  that  the  amount 
of  this  white  substance  varies  in  the  different  species.  In  most 
hybrids,  except  Fx  H179,  there  is  more  spreading  in  normal  abscission 
than  in  pure  species.  In  Nicotiana  quadrivalvis,  N.  Bigelovii,  and 
other  similar  species  in  which  abscission  very  seldom  occurs,  no  spread- 
ing takes  place.  Spreading,  however,  occurs  to  a  remarkable  extent  in 
N.  Tabacum  "Maryland." 

ft.  LYCOPEBSICUM 

We  may  say  that,  in  general,  abscission  in  Ly coper sicum  corre- 
sponds to  that  in  Nicotiana  and  that  the  main  points  of  distinction 
between  the  two  arise  only  from  the  original  differences  in  the  separ- 
ation zones  (cf.  page  364).  In  addition,  attention  must  be  called  to 
the  fact  that  quite  frequently,  in  individual  plants  of  the  tomato,  no 
true  abscission  occurs  in  normal  flower-fall.  In  these  cases  the  flower 
seems  to  be  detached  from  the  plant  by  a  process  which  compares 
closely  with  that  called  exfoliation.  There  is  no  active  cell  separation 
and  the  flower  simply  wilts  and  dries  back  to  the  groove,  where  it 
hangs  until  broken  off  by  some  mechanical  agency.  The  first  indica- 
tion of  the  process  is  the  loss  of  chlorophyl  in  the  pedicel,  which 
gradually  turns  yellow,  commencing  at  the  tip  and  spreading  proximal 
to  the  separation  zone.  It  is  possible  that  most  of  the  flower-fall 


374  University  of  California  Publications  in  Botany         [V°L-  5 

noticed  by  agriculturists  is  of  this  type.  Quite  often,  however,  true 
abscission  and  this  second  type  of  flower-fall  may  both  be  found 
operative  in  the  same  plant  or  even  in  the  same  flower.  "Spon- 
taneous" flower-fall  in  the  tomato  is,  of  course,  of  the  true  abscission 
type. 

Corresponding  with  the  condition  in  Nicotiana,  true  abscission  in 
Lycopersicum  is  seen  to  originate  frequently  in  the  pith.  At  any  rate, 
the  process  goes  on  here  independently  of  that  in  the  cortex,  since  the 
final  break  is  through  the  tracheae  and  epidermis.  Furthermore,  separ- 
ation takes  place  in  a  plane  with  the  bottom  of  the  groove  (pi.  53,  fig. 
2)  whereas,  in  Nicotiana,  it  takes  place  a  short  distance  distal  to  the 
groove.  Separation  may  at  first  take  place  between  only  two  tiers  of 
cells  (pi.  53,  fig.  2),  but  in  time  the  process  may  spread  until  three 
or  four  tiers  become  involved  in  separation.  However,  there  is  no 
spreading  of  the  process  to  a  large  number  of  cells,  as  is  frequently 
seen  in  Nicotiana,  so  that  one  very  seldom  finds  the  white  powdery 
substance  at  the  point  of  separation.  Also  in  contrast  with  the  con- 
dition in  Nicotiana,  there  is,  as  abscission  progresses,  no  bulging  of 
the  epidermis  which  instead  soon  breaks  in  the  bottom  of  the  groove. 
Separation  in  the  tomato  takes  place  in  such  a  way  as  to  give  the 
exposed  separation  surfaces  the  same  general  shape  after  abscission  as 
in  Nicotiana,  that  of  the  detached  portion  of  the  pedicel  being  convex 
and  that  of  the  remaining  portion  slightly  concave. 

c.  DATURA 

Conditions  in  Datura  differ  strikingly  from  those  in  the  two 
species  described  above.  This  would  be  expected  when  one  considers 
the  great  differences  in  the  structure  of  the  separation  zones  (cf. 
page  365).  In  Datura  there  is  the  usual  chlorophyl-bearing  tissue, 
which  consists  of  two  rows  of  small,  perfectly  isodiametric  cells  with 
large  intercellular  spaces,  just  beneath  the  epidermis.  It  will  be 
remembered  from  the  description  on  page  365  that  this  tissue  in  Datura 
continues  the  entire  length  of  the  pedicel  and  therefore,  in  contrast 
with  the  condition  in  Nicotiana  and  Lycopersicum,  extends  through 
the  separation  zone.  The  first  sign  of  abscission  is  the  maceration  of 
this  tissue  as  indicated  by  the  appearance  of  a  white  color  under  the 
epidermis.  The  latter  may  as  a  result  become  detached  from  the  tissues 
of  the  cortex  for  a  distance  of  2  cm.  or  more  along  the  base  of  the 
pedicel.  This  is  soon  followed  by  a  break  over  the  separation  layer 
and  a  curling  back  of  the  epidermis  on  either  side,  with  most  of  the 
chlorophyl-bearing  cortical  tissues  still  attached  to  its  inner  surface. 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae     375 

After  the  break  in  the  epidermis  separation  continues  in  the  layer 
of  collenchyma  just  beneath.  The  cells  of  the  collenchyma  layer, 
which  are  much  elongated  parallel  to  the  long  axis  of  the  pedicel  (five 
to  eight  times  as  long  as  wide),  separate  for  a  distance  of  about  0.3 
mm.  up  and  down  the  pedicel,  involving  only  a  few  tiers  of  cells.  It 
is  evident  that  the  cells  of  this  tissue  separate  without  difficulty, 
although  not  by  any  means  as  freely  as  the  small  spherical  cells  de- 
scribed above.  The  large,  isodiametric,  parenchyma  cells  of  the  cortex 
separate  for  a  distance  of  2  or  3  mm.,  involving  many  tiers  of  cells. 
The  cells  of  the  starch  sheath,  which  are  small  and  spherical,  separate 
for  a  distance  of  1  cm.  or  more,  thus  causing  a  longitudinal  cavity  to 
be  formed  just  outside  of  the  vascular  bundles.  In  the  latter,  separa- 
tion involves  only  two  or  three  tiers  of  cells.  Separation  originates 
and  continues  in  the  pith  independent  of  the  process  in  the  cortex, 
but  involves  about  the  same  number  of  cells  as  in  the  parenchyma  of 
the  latter  tissue.  "When  separation  has  thus  become  complete,  the 
weight  of  the  flower  is  very  often  sufficient  to  break  the  tracheag  and 
cause  the  flower  to  fall  to  the  ground. 

Several  important  facts  are  brought  out  by  this  examination  of 
abscission  in  Datura.  In  the  first  place,  it  shows  that  floral  abscission 
can  take  place  without  any  structure  which  might  possibly  be  inter- 
preted as  a  morphologically  differentiated  separation  layer.  In  the 
second  place,  it  indicates  that  cell  separation  is  possible  in  several  dif- 
ferent types  of  living  cells.  It  also  shows  that  separation  takes  place 
more  readily  in  small  cells  than  in  large  ones  and  more  readily  in 
isodiametric  cells  than  in  elongated  ones.  The  theory  that  the  separa- 
tion layer  is  not  a  morphologically  differentiated  structure,  but  repre- 
sents a  physiological  condition  (Lloyd  and  Loewi),  could  certainly  be 
well  applied  in  this  case. 

d.  OTHER  GENEEA 

The  process  of  abscission  in  the  other  species  listed  on  page  365  is 
essentially  the  same  throughout.  No  indications  were  noted  of  cell 
divisions  or  elongations  accompanying  abscission.  Separation  is 
brought  about  by  means  of  a  separation  of  small  and  active  cells 
located  in  the  general  region  at  the  base  of  the  pedicel.  In  all  these 
forms  the  separation  surface  of  the  pedicel  is  convex  in  outline,  so 
that  the  separation  layer  must  lie  in  more  or  less  of  a  crescent  in  the 
stem  at  the  base  of  the  pedicel.  The  main  difference  between  these 
forms  and  the  three  that  have  been  described  in  detail  above  is  found 


376  University  of  California  Publications  in  Botany         LV°L-  5 

in  the  fact  that  in  the  former,  with  the  exception  of  Solatium  tuber- 
osum,  separation  occurs  in  the  stem,  at  the  very  base  of  the  pedicel, 
whereas  in  the  latter  three  it  occurs  through  the  pedicel  a  varying 
distance  from  the  base. 

2.  METHOD  OF  CELL  SEPARATION 

a.  GENERAL  REMARKS 

It  will  be  remembered  that  two  theories  have  been  proposed  to 
account  for  the  cell  separation  that  is  responsible  for  abscission. 
First,  it  is  conceivable  that  cell  separation  may  be  caused  by  an 
increase  in  cell  turgor,  which  causes  the  cells  to  round  up  and  pull 
apart  without  any  change  taking  place  in  the  chemical  nature  of  the 
middle  lamella.  Second,  cell  separation  may  be  caused  by  a  chemical 
dissolution  of  the  middle  lamella  with  or  without  an  increase  in  cell 
turgor.  The  main  difference  between  the  two  theories  is  that  the 
second,  in  contrast  with  the  first,  maintains  that  chemical  alteration 
of  the  middle  lamella  is  always  necessary  before  abscission  can  occur. 
The  first  theory  gains  support  from  the  work  of  Pitting  and  the  second 
from  the  work  of  Hannig,  Lee,  Strasburger,  and  Lloyd.  Wiesner, 
Kubart,  and  Loewi  believe  that  cell  separation  takes  place  by  the 
action  of  both  factors  but  that  either  factor  may  at  times  be  the  more 
important. 

I.  CYTOLOGICAL   CHANGES  ACCOMPANYING   ABSCISSION 

It  was  stated  in  a  preliminary  discussion  (Goodspeed  and  Ken- 
dall, 1916)  first,  that  no  indication  of  cell  divisions  or  elongations 
were  observed  accompanying  abscission,  and,  second,  that  no  evidence 
of  the  dissolution  of  the  middle  lamella  had  at  that  time  been  obtained. 
The  first  statement  has  been  corroborated  in  that,  during  all  the  later 
experiments,  no  divisions  or  elongations  have  been  observed  in  any  of 
the  described  species.  The  dissolution  of  the  primary  cell  membrane, 
however,  because  of  more  exact  knowledge  of  the  proper  time  to  take 
sections  and  of  more  successful  staining  methods,  has  been  fairly  well 
established. 

The  main  problem  here  was  to  determine  by  the  use  of  various 
stains  whether  or  not  the  primary  and  secondary  cell  membranes  of 
the  separation  cells  stain  differently  in  the  early  stages  of  abscission 
than  under  normal  conditions.  This  was  a  point  which  was  found 
very  difficult  to  determine,  principally  because  of  the  fact  that  the 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    377 

separation  cells  are,  comparatively  speaking,  very  small,  but  also  be 
cause  of  the  fact  that  the  walls  of  these  cells  fail  to  show  any  strati- 
fication. 

Iodine,  Delafield's  haematoxylin,  Ruthenium  red,  Bismark  brown, 
methylene  blue,  erythrosin,  and  eosin  were  used  with  little  success  in 
most  cases.  By  using  iodine,  however,  just  as  abscission  is  known  to  be 
commencing,  a  white  streak  may  be  seen  across  the  section  in  the  region 
of  the  separation  layer.  Upon  careful  examination  it  was  decided 
that  this  white  streak  was  due  to  the  failure  of  most  of  the  cell  walls 
in  the  separation  layer  to  take  the  stain.  Although  it  is  probable 
that  with  more  careful  examination  the  other  stains  mentioned  above 
would  give  similar  results,  it  was  found  that  methylene  blue  was  the 
only  stain  with  which  anything  definite  could  be  established.  If  a 
thin  longitudinal  section  cut  in  paraffin  as  abscission  is  known  to  be 
starting,  and  stained  in  methylene  blue,  is  examined  (cf.  page  361),  it 
will  be  found  that  the  walls  of  those  cells  in  which  separation  is  about 
to  occur  have  remained  almost  entirely  unstained.  The  protoplasts  in 
these  cases  seem  to  be  surrounded  only  by  the  thin  tertiary  mem- 
branes, between  which  is  a  streak  of  colorless  material  of  varying 
width  (pi.  51).  Cell  walls  where  separation  is  not  expected  to  occur, 
however,  stain  a  dark  blue  throughout  in  the  normal  manner. 

An  examination  of  freshly  isolated  cells  washed  oft2  from  the  end 
of  an  abscissed  pedicel  shows  that  these  cells  are  still  turgid  and 
active.  It  was  impossible  to  determine  whether  these  cells  had  in- 
creased in  size,  as  compared  with  the  size  of  similar  cells  before  abscis- 
sion, but  it  is  evident  that  the  increase,  if  any,  had  not  been  very 
great.  The  cells  still  contain  their  large  nuclei,  and  occasional  starch 
grains,  and  show  after  isolation  no  signs  of  degeneration  even  after 
several  hours  in  water.  In  addition,  these  isolated  cells  appear  to  have 
retained  their  original  shape.  In  the  collenchyma  of  Datura  the  cells 
are  from  five  to  eight  times  as  long  as  wide,  and  yet  these  cells  retain 
their  original  shape  when  isolated,  as  a  result  of  the  dissolution  of 
the  middle  lamellae.  This  isolation  has  evidently  not  been  complete, 
since  large  masses  of  cells  are  seen  still  attached  to  each  other.  It  is 
noticed  that  in  all  cases  the  protoplast  is  surrounded  by  an  extremely 
thin  membranous  wall  (pi.  52,  fig.  3).  It  is  also  frequently  noticed 
that  the  protoplast  seems  drawn  away  from  the  cell  wall  as  if  plas- 
molysis  had  occurred.  It  is  possible  that  this  appearance  may  be  due 
simply  to  the  gathering  together  of  granules  and  the  denser  portion 
of  the  protoplasm  in  the  center  of  the  cell. 


378  University  of  California  Publications  in  Botany         [VOL.  5 


c.  EXPEEIMENTAL  EVIDENCE  FOE  THE  DISSOLUTION  OF 
THE  MIDDLE  LAMELLA 

It  is  supposed  that  the  middle  lamella,  or  primary  cell  membrane, 
is  largely  composed  of  calcium  pectate,  a  calcium  salt  of  pectic  acid 
which  has  been  given  the  general  name  pectose.  The  secondary  cell 
membranes  probably  contain  a  larger  proportion  of  cellulose  with 
the  pectose  than  is  present  in  the  primary  membranes.  This  pectose, 
which  is  of  course  insoluble  in  water,  is  disorganized  by  a  process  of 
hydrolysis  to  form  pectin.  The  pectin,  which  is  a  colorless  mucilagi- 
nous substance,  is  readily  soluble  in  water  but  is  precipitated  along 
with  the  proteids  and  enzymes  of  the  protoplast  by  the  addition  of 
alcohol.  Thus,  if  a  water  extract  is  made  from  separation  zones  dur- 
ing the  first  stages  of  abscission,  one  would  expect  to  get  a  solution  of 
several  substances,  among  which  would  be  the  pectin  produced  by  the 
dissolution  of  the  pectose  in  the  primary  cell  membranes.  It  might 
be  expected  that  the  amount  of  precipitate  obtained  from  this  extract 
with  alcohol  would  be  greater,  provided  the  amount  of  other  sub- 
stances remained  the  same,  than  the  amount  of  precipitate  obtained 
in  a  similar  manner  from  separation  zones  in  which  there  had  been 
no  abscission  and  in  which  no  pectin  had  been  formed.  Whether  or 
not  the  increase  in  the  amount  of  precipitate  is  due  to  the  added 
pectin  cannot  of  course  be  proven  without  actual  chemical  analysis, 
and  such  an  analysis  would  be  difficult  because  of  the  very  small  sam- 
ples of  material  obtainable.  However  this  may  be,  any  difference  in 
the  amount  of  precipitate  would  be  of  interest. 

This  experiment  and  the  two  which  follow  are,  as  far  as  I  have 
been  able  to  determine,  the  first  of  their  kind.  Apart  from  this  fact, 
their  chief  value  probably  lies  in  the  fact  that  they  suggest  a  line  of 
investigation  which,  if  carried  on  in  more  detail  and  with  better 
facilities,  will  undoubtedly  lead  to  important  conclusions.  These 
experiments  were,  however,  carried  on  with  as  much  care  as  possible 
and  since  the  results  of  duplicate  tests  are  in  agreement,  they  give,  as 
far  as  they  go,  dependable  results. 

After  several  experiments  indicating  the  results  given  below,  the 
following  test  experiment  was  performed: 

Experiment  1. — Two  water  extracts  of  equal  concentration  were 
made  from  the  lots  of  material.  Lot  A  contained  200  small  pieces  of 
the  pedicel  in  which  the  separation  zone  was  located  and  in  which 
abscission  had  started.  Lot  B  contained  an  equal  weight  of  a  similar 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    379 

number  of  pedicels  in  which  no  abscission  had  started.  The  extracts 
were  made  up  to  10  cc.  and  the  precipitate  obtained  with  60  cc.  of 
95  per  cent  alcohol.  The  precipitate  weighed  in  the  two  lots : 

A 996  nig. 

B : 903  mg. 

One  of  the  preliminary  experiments  performed  with~~a  "weaker 
alcohol  gave  results  which  may  or  may  not  be  of  considerable  impor- 
tance. In  this  experiment  a  light,  almost  invisible  precipitate  formed 
in  A  and  no  precipitate  in  B.  Whether  or  not  the  pectins  precipitate 
in  lower  percentages  of  alcohol  more  readily  than  the  other  substances 
I  have  been  unable  to  determine.  At  any  rate,  the  precipitate  in  this 
case  felt  slimy  and  mucilaginous  to  the  touch  and  might  well  have 
been  the  precipitated  pectin  approximately  pure. 

d.  EVIDENCE  FOE  INCEEASE  IN  TUEGOE 

It  was  stated  along  with  other  conclusions  in  the  preliminary  paper 
(Goodspeed  and  Kendall,  1916)  that  from  the  evidence  at  that  time 
available  it  was  probable  that  cell  separation  is  caused  merely  by  an 
increase  in  cell  turgor,  and  throughout  this  later  work  it  has  been 
clear  that  increased  turgor  is  present  during  abscission.  In  view  of 
the  evidence  given  above,  however,  it  would  seem  that  turgor  can 
play  only  a  secondary  role,  although  the  occurrence  of  increase  in 
turgor  must  not  be  ignored. 

The  bulging  of  the  epidermis  frequently  noted  as  accompanying 
abscission  is  evidence  of  increased  internal  pressure.  In  the  pith  the 
cells  next  to  those  which  are  separating  are  in  a  collapsed  condition 
due  to  the  pressure  of  the  expanding  separating  cells.  By  various 
experiments  it  can  be  shown  that  humid  conditions  favor  and  severe 
drought  prevents  abscission.  Richter  and  others  have  shown  that 
narcotic  vapors  which  cause  abscission  also  cause  increased  turgor  by 
increasing  the  proportion  of  sugar  in  starch-containing  cells.  This 
increase  in  cell  turgor  becomes  so  great  as  to  cause  complete  macera- 
tion in  certain  types  of  tissues.  The  frequent  presence  of  starch 
grains  in  the  separation  layer  of  Nicotiana,  part  of  which  are  prob- 
ably converted  into  sugar  as  a  result  of  subjection  to  illuminating 
gas,  indicates  that  there  is  probably  an  increase  of  turgor  during 
abscission,  at  any  rate  when  induced  by  illuminating  gas. 

On  the  other  hand,  a  more  extensive  examination  of  abscission  in 
certain  plants  indicates  that  all  evidences  of  increased  turgor  may  at 


380  University  of  California  Publications  in  Botany         [VOL.  5 

times  be  absent.  Such  cases  might  be  explained  by  the  absence  of 
any  considerable  amount  of  starch  in  the  cells  concerned.  Indeed, 
the  starch  grains  usually  noted  in  the  separation  layer  can  not  at 
times  be  observed.  This  might  also  explain  the  fact  that  the  bulging 
of  the  epidermis  and  collapse  of  cells  in  the  pith  usually  accompany- 
ing abscission  are  sometimes  absent.  Also,  starch  grains  are  rarely 
observed  in  the  separation  cells  of  Lycopersicum  and  Datura  and  in 
these  forms  very  little  bulging  of  the  epidermis  occurs.  Although 
humid  conditions  favor  abscission  and  drought  prevents  the  process, 
it  has  also  been  observed  that  drought  has  to  be  very  severe  before  it 
produces  such  a  result.  Other  evidences  for  increased  turgor  derived 
from  the  turgid  appearance  of  the  cells  are  mostly  obtained  after 
abscission  has  started  and,  granting  that  the  cells  are  isolated  by  dis- 
solution of  the  middle  lamella,  more  or  less  expansion  due  to  release 
of  pressure  is  to  be  expected. 

A  critical  examination  of  the  separation  cells  during  abscission 
brings  out  several  facts,  other  than  those  mentioned  in  the  above 
paragraph,  which  of  themselves  render  inadmissible  the  theory  that 
cell  separation  is  brought  about  by  increased  turgor.  These  are 
as  follows:  1.  There  seems  to  be  no  perceptible  change  in  cell 
shape  or  size  during  separation.  2.  The  increase  in  size  of  the  inter- 
cellular spaces  does  not  necessarily  take  place  first  between  the  walls 
at  the  " corners"  of  the  cells,  but  may  appear  first  as  a  longitudinal 
streak  between  the  lateral  walls  of  the  cells  (pi.  51).  3.  Cell  isola- 
tion may  be  incomplete  in  large  numbers  of  cells  still  remaining 
attached  to  each  other.  4.  Cell  separation  first  becomes  complete  in 
a  narrow  plane  between  only  two  tiers  of  cells  before  spreading  later 
to  a  larger  number  of  cells.  5.  The  spreading  of  cell  separation 
itself  is  obviously  hard  to  explain  on  the  basis  of  the  turgor  theory. 

In  view  "of  the  facts  brought  out  in  this  discussion  and  the  positive 
evidence  for  the  dissolution  of  the  primary  membranes,  it  should  be 
clear  that  increase  in  turgor,  at  least  in  the  Solanaceae,  is  not  the 
direct  cause  of  cell  separation.  Undoubtedly  there  is  often  great 
increase  in  turgor  during  abscission,  especially  in  certain  types  of 
cells,  but  this  increase,  instead  of  being  the  direct  initiating  factor, 
probably  serves  merely  to  hasten  and  facilitate  the  process. 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanacette    381 


e.  EXPERIMENTS  ON  THE  AMOUNT  OF  SUGAR  IN  THE  STEM 
AND  PEDICEL  OF  NICOTIAN  A  DURING  ABSCISSION 

After  several  experiments,  all  of  which  indicated  the  results  ob- 
tained below,  the  following  experiment  was  performed.  Experiment 
2a  was  devised  to  show  the  change  in  the  amount  of  sugar  which 
occurs  in  the  tissues  of  the  pedicel  during  abscission.  Experiment  2& 
was  intended  to  show  this  same  difference  in  a  restricted  region  of  the 
stem  just  proximal  to  the  separation  layer. 

Experiment  2a.  Lot  A  included  200  pedicels  of  flowers  which  had 
fallen  a  few  minutes  before  being  collected  as  a  result  of  being  sub- 
jected to  illuminating  gas.  Lot  B  included  200  pedicels  of  flowers 
picked  at  the  same  time  as  those  making  up  Lot  A,  but  in  which  no 
abscission  was  induced.  The  water  extracts  made  with  10  cc.  from 
equal  weights  of  the  two  lots  were  tested  with  surplus  Fehlings  solu- 
tion. The  precipitates  formed  upon  boiling  weighed : 

A  68  mg. 

B  95  mg. 

Experiment  21}.  This  experiment  was  carried  out  in  the  same  man- 
ner as  experiment  2a,  but  the  precipitates  in  this  case  were  of  such 
small  quantity  that  no  attempt  was  made  to  get  actual  figures  as  to 
their  weights.  It  was  clear,  however,  merely  from  an  examination  of 
the  filter  paper,  that  there  was  more  precipitate  in  B  than  in  A — just 
the  reverse  of  Experiment  2a.  The  difference  was  evidently  not  as 
great  as  in  the  latter  experiment. 

Experiment  2a  seems  to  indicate  that  during  abscission  there  is  a 
reduction  of  nearly  one-third  the  normal  amount  of  sugar  in  the 
pedicel.  Other  preliminary  experiments  performed  as  abscission  was 
starting  showed  only  a  slight  reduction  in  the  amount  of  sugar  in  the 
pedicel.  Thus  possibly  the  withdrawal  of  sugar  commences  with  the 
start  of  abscission.  Experiment  2&  indicates  that  there  is  probably  a 
slight  increase  in  the  amount  of  sugar  in  the  limited  region  proximal 
to  the  separation  during  abscission.  It  is  possible  that  most  of  the 
withdrawn  sugar  is  used  as  a  source  for  the  energy  required  in  the 
active  process  of  cell  separation.  The  slight  increase  proximal  to  the 
separation  layer  also  shows  that  there  is  probably  an  increase  in  cell 
turgor  in  the  actual  tissues  which  contain  the  separation  layer,  due 
to  the  conversion  of  starch  into  sugar. 


382  University  of  California  Publications  in  Botany         [VOL.  5 


/.  POSSIBLE  AGENCY  ACTIVE  IN  THE  DISSOLUTION  OF 
THE  MIDDLE  LAMELLA 

The  pectose  of  the  middle  lamella  may  be  broken  down  into  the 
soluble  pectin  in  three  different  ways — by  the  action  of  an  acid, 
of  an  alkali,  or  of  the  enzyme  pectosinase.  Since  it  is  doubtful 
whether  alkaline  reactions  in  living  cells  frequently  get  strong  enough 
to  affect  the  middle  lamella,  the  probable  active  agency  is  limited 
to  the  acid  or  the  enzyme  action.  Up  to  the  last  few  years  very 
little  has  been  known  about  the  action  of  enzymes  concerned  in 
pectic  digestion.  It  has  been  natural,  therefore,  for  investigators 
(cf.  Wiesner,  1905,  and  Kubart,  1906)  to  consider  the  acid  as  prob- 
ably the  active  agency.  In  this  connection,  it  is  well  to  state  that  I 
have  obtained  distinct  acid  reactions  with  litmus  from  the  base  of  the 
corolla  of  Nicotiana  during  abscission.  This  would  confirm  Kubart, 
who,  it  will  be  remembered,  obtained  similar  reactions  from  the  corolla 
of  Nicotiana.  But  in  this  case  I  sometimes  obtained  acid  reactions 
from  the  corolla  when  in  the  normal  condition.  Since  these  observa- 
tions offer  no  detailed  evidence  that  acidity  has  increased  during 
abscission  to  a  degree  higher  than  normal,  their  significance  can  well 
be  doubted. 

The  tissues  of  Datura  give  a  distinct  acid  reaction  to  litmus  in  the 
normal  condition.  Experiment  3  below  shows  a  slight  increase  in 
acidity  during  abscission.  No  acid  reactions  of  much  intensity  are 
given  by  the  base  of  the  pedicel  of  Nicotiana  either  in  the  normal  or 
abscissed  condition. 

Experiment  3.  Lot  A  contained  the  bases  of  three  pedicels  cut 
while  abscission  was  going  on.  Lot  B  contained  an  equal  weight 
(6  gin.)  of  the  bases  of  three  pedicels  cut  in  the  normal  condition. 
These  were  extracted  with  water  and  the  extracts  made  up  to  10  cc. 
each.  By  titration  with  10  per  cent  NaOH  and  phenolphtalein  the 
following  results  were  obtained : 

A 0.75  cc.  required  to  neutralize 

B 0.6     cc.  required  to  neutralize 

A  similar  experiment  on  Nicotiana  showed,  however,  that  the  nor- 
mally low  acidity  of  this  genus  is  slightly  reduced  during  abscission, 
as  indicated  by  the  following  results: 

A 0.25  cc.  required  to  neutralize 

B 0.37  cc.  required  to  neutralize 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceac     383 


The  normal  acidity  in  Datura  is  high,  but  it  is  doubtful  whether 
the  increase  is  large  enough  to  account  for  the  dissolution  of  the 
middle  lamella.  At  any  rate,  it  is  certain  that  acidity  does  not  enter 
into  the  problem  in  the  pedicel  of  Nicotiana.  We  must,  therefore, 
fall  back  upon  the  enzyme  action  as  probably  responsible  for  the 
process  of  cell  separation. 

Most  hydrolysing  processes  characteristic  of  living  cells  aFe  now 
supposed  to  be  due  to  the  action  of  enzymes  of  different  kinds.  It 
has  been  definitely  claimed  (cf.  Atkins,  1916)  that  an  enzyme  which 
has  been  called  pectosinase  is  capable  of  breaking  down  the  pectose 
of  which  the  middle  lamella  is  composed.  Add  to  this  the  fact  that  the 
action  of  enzymes  has  been  shown,  as  has  also  the  process  of  abscission, 
to  be  very  sensitive  to  all  kinds  of  changes  in  the  external  environment, 
and  it  is  fairly  safe  to  assume  that  the  method  of  cell  separation  is 
fundamentally  an  enzyme  problem.  Irrefutable  proof  of  this  could 
be  obtained  only  by  testing  for  the  activity  of  pectosinase  during  the 
early  stages  of  abscission  and  by  demonstrating  the  absence  or  in- 
activity of  this  enz3rme  in  species  where  abscission  does  not  occur. 

ABSCISSION  OF  THE   STYLE  AND   COROLLA 

Abscission  of  the  corolla  in  Nicotiana  was  described  by  Kubart 
and  it  may  be  said  at  once  that  the  observations  herein  described 

agree  entirely  with  his.  Abscission  of  the 
corolla  is  brought  about  by  the  separation, 
without  any  previous  cell  divisions  or 
elongations,  of  living  cells  at  the  base  of 
the  corolla  tube.  The  separation  layer, 
which  is  in  no  way  morphologically  differ- 
entiated from  the  neighboring  tissue,  is 
located  about  1  mm.  from  the  point  of  in- 
sertion of  the  corolla  on  the  receptacle.  It 
thus  occurs  in  the  distal  part  of  a  region 
where  intergradation  of  cell  shape,  between 
the  isodiametric  cells  of  the  receptacle 
and  the  more  or  less  elongated  cells  of  the 
corolla,  is  apparent.  The  separation  cells 
which  are  in  this  region  of  intergradation 

Fig.  6.    Longitudinal  radial  .  . 

section  of  the  base  of  the    are  not  isodiametric  but  are  more  or  less 

corolla    tube    of    Nicotiana,    elongated  parallel  to  the  long  axis  of  the 
showing  the   method   of   ab- 
scission, corolla.    All  the  cells  in  cross-section  of  the 


384 


University  of  California  Publications  in  Botany         [VOL.  5 


base  of  the  corolla  tube  at  about  the  level  of  the  separation  layer  seem 
to  be  involved  in  the  process  except  the  epidermal  cells  and  the 
tracheae.  The  process  of  cell  isolation  in  this  case  may  spread  up  and 
down  for  quite  a  distance  between  the  epidermis  and  tracheae,  thus 
involving  a  large  number  of  cells  (fig.  6). 

Abscission  of  the  corolla  in  Datura  differs  slightly  from  that  in 
Nicotiana,.  As  in  the  latter,  there  is  no  differentiated  separation 
layer,  separation  occurring  in  cells  which  are  not  visibly  different 
from  other  cells  of  the  corolla.  Cells  more  or  less  elongated  are  in- 
volved, as  in  Nicotiana,  but  in  Datura  the  region  of  separating  cells 
is  limited  to  certain  tissues — that  is  to  say,  not  all  the  cells  across 
the  base  of  the  corolla  tube  at  about  the  level  of  the  separation  layer 
are  involved  in  the  process  of  abscission.  The  base  of  the  corolla  in 
Datura  is  characterized  by  distinct  longitudinal  ridges  which  alternate 
with  deep  grooves.  Thus,  a  cross-section  of  a  portion  of  the  base  of 
the  corolla  appears  as  in  fig.  8.  Cell  separation  fails  to  occur  in  the 
outside  ridges  at  the  level  of  the  separation  layer,  so  that,  looking  at 
the  base  of  the  corolla  tube  from  the  outside  during  abscission,  one 
sees  separate  crescent-shaped  regions  of  macerating  cells  alternating 
with  cells  which  are  not  separating  (fig.  7).  This  is  explained  when 
a  cross-section  is  taken  (fig.  8),  which  shows  that  several  vascular 
bundles,  the  cells  of  which  do  not  sep- 
arate, are  collected  in  the  outside  ridges. 

Abscission  of  the  style  occurs  nor- 
mally in  Nicotiana  and  Datura  a  short 
time  before  the  corolla  has  fallen.  So 
far  as  it  was  possible  to  determine,  the 
process  of  abscission  is  exactly  the  same 
in  the  style  as  in  the  corolla.  A  separa- 
tion of  very  small,  more  or  less  elongated 
cells  takes  place  at  the  base  of  the  style 
without  any  external  indication  such  as  Fig.  7 

frequently  occurs  in  the  pedicel  of  the      c— region  of  separating  cells. 


'a    c 
Fig.  8 

a — vascular  bundle. 
c — region  of  cell  separation. 
5 — region  of  no  cell  separation, 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    385 

flower.  As  in  the  case  of  the  corolla,  there  is  no  structure  which  might 
be  interpreted  as  a  differentiated  separation  layer.  Separation  occurs 
in  a  region  of  intergradation  in  cell  shape  between  the  spherical  cells 
of  the  ovary  and  the  cells  of  the  style,  which  are  elongated  parallel 
with  the  long  axis  of  the  style. 

TIME  OF  ABSCISSION 
1.  REACTION  TIME 

a.  EEACTION  TIME  IN  NOKMAL  ABSCISSION 

The  term  "reaction  time"  is  used  in  referring  to  two  rather  dis- 
tinct subjects.  First,  we  may  have  a  reaction  time  represented  by 
the  time  intervening  between  anthesis  and  normal  abscission  due  to 
lack  of  fertilization.  Second,  we  may  have  a  reaction  time  which  has 
to  do  with  the  period  between  the  application  of  the  stimulus  and 
flower-fall  in  ' '  spontaneous ' '  abscission.  The  reaction  times  in  normal 
abscission  were  discussed  in  an  earlier  communication  in  the  case  of 
two  F!  species  hybrids  of  Nicotiana  and  their  parents.  The  state- 
ments there  made  have  been  repeatedly  verified  and  in  addition  a 
considerable  amount  of  data  has  been  accumulated  in  regard  to  the 
time  of  abscission  in  other  species  of  Nicotiana  and  in  the  genus 
Lycopersicum.  In  the  case  of  the  former  the  observations  were  also 
made  upon  the  abscission  of  the  corolla,  the  effect  of  pollination  on 
reaction  time,  and  the  reaction  time  in  "spontaneous"  abscission. 

In  determining  the  abscission  times  for  the  hybrid  Ft  H154  (cf. 
page  386) ,  a  great  variation  was  noted  in  the  normal  reaction  time.  In 
the  case  of  the  hybrid  Fj  H179  (cf.  page  386),  however,  and  in  other 
species  or  varieties  investigated,  very  little  variation  in  the  time  of 
abscission  has  been  noted.  The  range  of  variation  in  these  species 
and  varieties  practically  always  falls  within  two  or  three  days  and  a 
large  number  of  observations  gives  identical  times,  as  far  as  the  number 
of  days  is  concerned,  in  the  case  of  seven  to  ten  flowers.  There  is  a  cer- 
tain variation  in  the  length  of  the  reaction  times  in  different  flowers  on 
the  same  plant,  but  the  plants  of  a  species  do  not  differ  from  one  an- 
other in  their  average  reaction  times.  It  was  noticed  that  the  figures 
were  approximately  the  same  whether  the  averages  were  based  on  the 
records  of  four  or  five  flowers  or  of  a  considerably  larger  number ; 
thus  the  results  given  in  the  following  table  may  be  considered  con- 
clusive. Where  the  number  of  flowers  involved  is  less  than  four, 


386 


University  of  California  Publications  in  Botany         {.VOL.  5 


however,  the  results  serve  merely  as  approximate  estimate  of  the 
abscission  reaction  time. 

In  obtaining  the  records  tabulated  below  a  separate  tag  was  sup- 
plied for  each  flower.  This  tag  was  put  on  the  flower  at  the  begin- 
ning of  the  observation,  the  plant  visited  twice  a  day  thereafter  and 
records  kept  on  the  tags,  which  were  left  on  the  flower  until  the  close 
of  the  observation.  If  a  flower  fell  upon  being  tapped  or  shaken, 
abscission  was  considered  to  have  occurred  and  the  date  was  recorded 
on  the  tag,  which  was  then  collected.  Similarly,  in  the  case  of  the 
records  for  abscission  of  the  corolla,  a  slight  pull  had  to  be  applied 
before  it  could  be  determined  whether  abscission  had  occurred.  As  a 
means  of  preventing  fertilization,  the,  stigma  was  cut  away  in  addi- 

TABLE  1 


I 

II 

III 

IV 

V 

Time  from 

Time  from 

Time  from 

Time  from 

Time  from 

bud1  to 
anthesis 

pollination  to 
mature  fruit 

pollination  to 
abscission  of 

anthesis  to 
abscission  of 

anthesis  to 
normal  flower3 

Designation  of 
species  or  variety 

corolla 

corolla, 
unpollinated 

fall 

No. 
flowers 

Avg. 
No. 
days 

No. 

flowers 

Avg. 
No. 
days 

No. 
flowers 

Avg. 

No. 
days 

No. 
flowers 

Avg. 
No. 

days 

No. 
flowers 

Avg. 
No. 
days 

Fi  H38 

10 

18 

Ft  H179 

4 

8 

4 

14 

15 

3 

23 

82 

6 
6 

20 

7 

Ft  H36 

7 

4 

3 

6 

N.  sylvestris        « 

10 

11 

6 

4 

9 

42 

6 
6 

9 

15 

N.  Tabacum 

3 

12 

2 

4 

2 

6 

3 

5 

"Maryland" 

N.  Bigelovii 

5 

10 

5 

3 

5 

5 

8 

17 

var.  Wallace! 

•N.  Bigelovii 

5 

19 

5 

2 

6 

5 

G 

no  fall 

"Porno" 

N.  Bigelovii          f 

0 

16 

var.  typica        \ 

3 

no  fall 

N.  Bigelovii 

4 

21 

1 

4 

2 

8 

(hybrid?) 

N.  multivalvis 

3 

18 

2 

4 

2 

no  fall 

N.  quadrivalvis 

5 

20 

5 

7 

5 

2 

6 

no  fall 

N.  suaveolens 

8 

8 

8 

8 

7 

13 

N.  Sanderae 

15 

15 

5 

4 

5 

6 

6 

9 

N.  rustica  var. 

3 

4 

4 

6 

brasilia 

N.  rustica 

7 

6 

7 

3 

4 

4 

(Winnebago) 

N.  rustica  var.? 

4 

15 

4 

2 

5 

3 

6 

5 

Lycopersicum 
esculentum 

4 

6 

4 

3 

4 

8 

17 

9 

1  The  buds  recorded  here  were  of  such  size  that  the  corolla  and  calyx  were  of  the 
same  length. 

2  Sterile  pollen  applied  to  the  stigma. 

8  By  normal  flower-fall  is  meant  fall  due  to  lack  of  fertilization. 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    387 

tion  to  removing  the  anthers  before  anthesis.  Various  experiments 
had  shown  that  such  an  operation  on  the  flower  does  not  induce 
abscission  or  affect  its  normal  physiological  condition  to  any  great 
extent. 

F!  H154  is  Nicotiana  Tabacum  var.  macrophylla  (U.  C.  B.  G. 
22/07)  X  A7-  sylvestris  (U.  C.  B.  G.  69/07).  F±  H179  is  N.  Tabacum 
"Cuba"  (U.  C.  B.  G.  200/14)  X  N.  sylvestris.  Ft  H36  i^N~rsylvestris 
X  N.  Tabacum  var.  angustifolia  (U.  C.  G.  B.  68/07). 

The  results  given  in  table  1  indicate,  in  the  first  place,  that  the 
different  species  differ  considerably  in  all  the  types  of  abscission 
reaction  times  considered,  and,  in  the  second  place,  that  on  the  aver- 
age, application  of  a  fertile  pollen  to  the  stigma  tends  to  shorten 
the  time  between  anthesis  and  abscission  of  the  corolla  by  two  days. 
The  one  apparent  exception  to  this  statement  is  Nicotiana  suaveolens, 
but  in  this  case  the  pollinated  flowers  fell  five  or  six  days  later 
than  the  corolla,  indicating  that  growth  of  the  pollen  had  not  pro- 
ceeded very  far.  Records  on  Fx  H179  and  N.  sylvestris  indicate  that 
sterile  pollen  does  not  have  the  same  effect  on  abscission  that  fertile 
pollen  does.  This  would  seem  to  show  that  here  the  effect  of  pollina- 
tion upon  the  postfloration  phenomena  is  not  due,  as  Fitting  (1909) 
has  found  in  orchids,  to  mere  mechanical  or  .chemical  stimulation  of 
the  stigma  by  the  pollen.  This  much  being  certain,  the  question  still 
remains  whether  the  results  obtained  depend  upon  fertilization  or  are 
due  to  the  growth  of  the  pollen  tubes  down  through  the  style. 

According  to  East  (1915),  working  on  self -sterility  in  Nicotiana 
hybrids,  the  pollen  tubes  reach  the  ovary,  in  cases  of  cross-pollination, 
three  or  four  days  after  application  of  pollen  to  the  stigma.  Since  in 
all  cases  recorded  above  cross-pollination  was  carried  on  and  since  in 
most  cases  the  corolla  was  not  thrown  off  until  three  or  four  days 
after  application  of  pollen  to  the  stigma,  it  is  possible  that  fertiliza- 
tion is  the  important  factor  in  shortening  the  time  between  anthesis 
and  abscission  of  the  corolla.  In  N.  qttadrivalvis,  however,  the  corolla 
was  thrown  off  within  eighteen  hours  after  pollination,  whereas,  when 
pollination  is  prevented,  the  corolla  may  remain  on  the  flower  for 
fifty-seven  hours.  If  East's  conclusions  are  correct,  this  would  seem 
to  indicate  that  the  shortening  of  the  reaction  time  in  abscission  of 
the  corolla  is  due  to  some  stimulation  of  the  style  by  the  pollen  tubes 
and  not  to  fertilization.  This  conclusion,  however,  could  be  doubted 
even  here,  because  the  style  of  N.  quadrivalvis  is  very  short,  so  that 
the  pollen  tubes  might  reach  the  ovary  in  a  much  shorter  time  than  is 


388  University  of  Californw  Publications  in  Botany         [Vol..  ~> 

required  in  larger  flowers.  An  attempt  was  made  to  get  further  data 
on  this  point  by  removing  the  style  several  Jiours  after  application  of 
pollen  before  the  pollen  tubes  could  possibly  have  reached  the  ovary. 
This  operation  occasionally  causes  the  whole  flower  to  fall,  and  since 
in  such  cases  abscission  in  the  pedicel  occurs  before  fall  of  the  corolla, 
no  results  in  regard  to  the  latter  organ  are  obtained.  The  possible 
effect  of  the  operation  on  the  abscission  of  the  corolla  was  checked 
by  control  tests  of  unpollinated  flowers  in  which  the  styles  had  also 
been  removed.  This  can  also  be  checked  by  a  comparison  with  the 
periods  of.  time  given  in  table  1,  column  III. 

It  was  found  in  three  flowers  of  ¥t  H179  that,  when  the  style  was 
removed  three  days  after  pollination,  the  corolla  was,  on  the  average, 
thrown  off  three  days  after  anthesis.  The  control  test  for  this  experi- 
ment gave  in  three  flowers  an  average  of  five  days.  Where  the  style' 
was  removed  two  days  after  anthesis,  four  flowers  gave  an  average  of 
three  days.  Where  the  style  was  removed  one  day  after  pollination, 
the  corolla  was  abscissed  in  five  flowers  an  average  of  three  days  after 
anthesis.  A  control  test  gave  in  this  case  an  average  of  five  days  for 
five  flowers.  Finally,  the  style  was  removed  in  seven  flowers  seventeen 
hours  after  pollination.  The  seven  flowers  gave  in  this  case  an  average 
of  four  days  for  the  time  between  anthesis  and  fall  of  the  corolla.  A 
control  test  for  this  last  case  gave  for  five  flowers  an  average  of  five 
days. 

These  experiments  were  repeated  with  N.  sylvestris.  In  one  case 
where  the  style  was  removed  in  three  flowers  two  days  after  pollina- 
tion, the  corolla  was  thrown  off  on  an  average  of  four  days  after 
anthesis.  A  control  test  of  this  case  gave  an  average  of  six  days  for 
three  flowers.  In  another  case  the  style  was  removed  in  three  flowers 
one  day  after  pollination.  In  this  case  the  corolla  was  abscissed  011 
an  average  of  three  days  after  anthesis. 

The  results  given  in  the  above  paragraphs  indicate  definitely  that 
it  is  the  stimulation  of  stylar  tissues  caused  by  the  growth  of  the 
pollen  tubes  which  shortens  the  time  between  anthesis  and  abscission 
of  the  corolla.  They  also  show  that  the  removal  of  the  style  has  no 
appreciable  effect  on  the  abscission  of  the  corolla.  It  is  evident  from 
the  results  given  that  the  influence  of  the  pollen  is  seen  as  early  as 
seventeen  hours  after  pollination,  and  it  is  possible  that  the  effect  may 
be  manifested  even  earlier.  It  is  significant  that  the  period  given  in 
the  case  where  the  style  was  removed  seventeen  hours  after  pollina- 
tion is  one  day  longer  than  in  the  case  where  the  style  was  removed 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    389 

twenty-four  hours  after  pollination.  This  may  possibly  indicate  that 
in  the  first  case  the  influence  of  the  pollen  tubes  has  diminished,  be- 
cause of  the  shortening  of  the  period  which  they  have  had  for  growth. 
If  this  is  the  case,  it  is  reasonable  to  suppose  that  the  influence  of  the 
growing  pollen  tube  increases  up  to  twenty-four  hours  after  pollina- 
tion as  the  pollen  tube  lengthens.  Thus,  at  six  hours  after  pollination 
it  is  possible  that  no  effect  of  the  pollen  tubes  would  be~ noticeable, 
while  twenty-four  hours  after  pollination  the  entire  influence  of  the 
growing  pollen  tube  has  been  exerted. 

The  effect  of  pollination  on  the  time  between  anthesis  and  flower- 
fall  was  tested  by  experiments  similar  to  those  described  above. 
Results  in  such  experiments  are  difficult  to  obtain  because  removal  of 
the  style  frequently  causes  the  premature  fall  of  the  flower.  If  the 
flower  fell  before  abscission  of  the  corolla,  the  fall  was  considered 
premature,  as  the  result  of  the  removal  of  the  style,  and  the  record  of 
that  particular  flower  not  considered.  Since  under  ordinary  condi- 
tions pollinated  flowers  remain  on  the  plant,  it  is  to  be  expected  that 
the  stimulation  of  the  stylar  tissues  by  the  pollen  tubes,  if  it  has  any 
influence  at  all,  would  increase  the  length  of  time  between  anthesis 
and  flower-fall.  Granting  the  truth  of  this  assumption,  any  reduction 
in  time  between  anthesis  and  fall  can  be  considered  as  the  result  of 
removal  of  the  style. 

In  one  test  on  ten  flowers  of  Fx  H179,  where  the  style  was  removed 
two  days  after  pollination,  flower-fall  occurred  on  an  average  of  seven 
days  after  anthesis.  A  control  test  in  this  case  also  gave  seven  days 
for  ten  flowers.  This  time  is  approximately  the  same  (the  actual 
average  calculated  to  the  tenth  of  a  day  was  6.7)  as  those  given  in 
table  1,  column  V,  for  the  time  between  anthesis  and  normal  flower- 
fall  due  to  lack  of  fertilization.  A  similar  test  on  six  flowers  of  N.  syl- 
veslris,  where  the  style  was  removed  two  days  after  pollination,  gave 
an  average  of  thirteen  days.  The  time  for  this  species  in  table  1, 
column  V,  is  fifteen  days.  »> 

These  two  records  indicate  that  the  stimulation  of  the  stylar  tissues 
by  the  growing  pollen  tubes  has  no  effect  on  the  time  between  anthesis 
and  flower-fall.  In  the  second  case  above,  and  also  perhaps  in  the 
first,  the  stimulation  of  the  style  seems  to  have  shortened  the  time 
somewhat,  but  in  this  case  the  result  can  be  explained  by  the  effect 
of  the  later  removal  of  the  style. 


390  rniuersity  of  California  Publications  in  Botani/          [VOL.  5 

b.  REACTION  TIME  IN  "SPONTANEOUS"  ABSCISSION 

Exact  data  in  regard  to  the  reaction  time  can  be  given  only  in 
two  definite  cases.  The  observations  in  these  cases  were  made  on 
small  shoots  of  the  plant  to  be  considered,  which  were  placed  in  water 
and  inserted  under  a  bell-jar  containing  1.5  per  cent  illuminating 
gas.  After  several  hours,  the  material  was  shaken 'every  fifteen  min- 
utes to  determine  when  the  first  flower  fell.  Fx  H179  and  N.  Tabacum 
1 '  Maryland "  were  selected  as  material  for  the  experiments  because 
these  forms  were  found  most  sensitive  and  thus  react  regularly  and 
quickly  to  stimuli.  Abscission  occurs  in  the  pedicel  of  Fx  H179  seven 
hours  after  insertion  into  1.5  per  cent  illuminating  gas  at  a  tempera- 
ture of  approximately  19°  C.  The  smaller  buds  begin  to  fall  first, 
but  are  followed  in  a  short  time  by  the  open  flowers.  Abscission 
occurs  in  X.  Tabacum  "  Mary  land"  in  eight  hours  under  the  above 
conditions. 

The  remainder  of  the  data  having  to  do  with  the  reaction  time  in 
spontaneous  abscission  is  in  the  form  of  approximate  estimates  derived 
from  the  results  of  experiments  on  the  induction  of  abscission.  In 
the  case  of  abscission  induced  by  illuminating  gas  most  species  which 
shed  their  flowers  in  1.5  per  cent  illuminating  gas  do  so  after  ten  or 
fifteen  hours  at  room  temperature. 

There  remains  now  to  be  considered  the  reaction  time  in  cases  of 
flower-fall  due  to  mechanical  injury.  The  results  along  this  line  are 
largely  derived  from  tables  2,  3,  4,  and  5,  which,  however,  were 
arranged  to  show  more  particularly  the  comparative  effect  of  different 
types  of  injury,  as  causing  or  not  causing  abscission  in  flowers  of 
various  ages.  These  tables  might  as  well  be  presented  under  the 
heading  "Experimental  Induction  of  Abscission  by  Mechanical  In- 
jury" (page  405),  but  since  it  is  necessary  to  draw  certain  conclusions 
from  them  in  regard  to  the  time  of  abscission  they  are  presented  and 
explained  at  this  time. 

Tables  2,  3,  4,  and  5,  which  follow,  serve  to  record  the  results  of 
a  number  and  variety  of  experiments  all  designed  to  show  the  relation 
of  mechanical  injury  to  abscission.  It  was  very  soon  discovered  while 
carrying  on  the  experiments  that  the  effect  of  injury  depends  to  a 
large  extent  upon  the  age  of  the  flower.  Now  the  age  of  the  flower 
can  be  most  conveniently  measured  by  determining  the  increase  in 
size  of  growing  parts  such  as  the  corolla  and  ovary.  Thus  it  was 
necessary  in  each  case  to  record  the  size  of  the  flower — size  being  a 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    391 

criterion  of  age — upon  which  the  test  was  being  made.  This  was  done 
by  noting  on  the  tag  which  was  supplied  for  each  flower  (cf.  page  385) 
the  length  of  the  corolla  in  millimeters,  the  condition  of  the  corolla, 
or  any  other  condition  of  the  flower  which  would  serve  to  indicate  its 
age.  The  period  of  development  of  the  flower  and  fruit  is  divided 
into  several  arbitrary  stages,  each  of  which  is  designated  by  a  Roman 
numeral  in  the  second  column  of  the  tables.  Where  the~mimber  of 
flowers  designated  in  the  first  column  are  nearly  in  the  same  stage 
of  development  only  one  numeral  appears  in  the  table,  but  where  the 
range  in  size  of  the  flowers  is  quite  extensive  two  numerals  appear, 
representing  the  range  in  size  within  which  the  flowers  were  found  at 
the  time  of  the  experiment.  The  stages  of  floral  development  which 
each  Roman  numeral  represents  are  given  below. 

Bud 

I corolla    2  mm.  to    5  mm.  in  length 

II corolla     6  mm.  to  10  mm.  in  length 

III corolla  11  mm.  to  15  mm.  in  length 

IV corolla  16  mm.  to  20  mm.  in  length 

V corolla  21  mm.  to  30  mm.  in  length 

VI corolla  31  mm.  to  40  mm.  in  length 

VII corolla  41  mm.  to  50  mm.  in  length 

Flower 

VIII corolla  opening 

IX anthesis 

X 2  days  after  anthesis 

XI corolla  withering 

Fruit 

Immature 

XII fruit     5  mm.  to     8  mm.  in  length 

XIII fruit     9  mm.  to  10  mm.  in  length 

Mature 
XIV fruit  11  mm.  to  12  mm.  in  length 

The  operation  of  injuring  the  flower  consisted  largely  in  removing, 
by  cutting  away  with  a  sharp  safety  razor  blade,  entire  floral  organs 
or  parts  of  them.  In  some  cases,  however,  organs  were  only  slit  longi- 
tudinally with  a  sharp  knife  or  merely  punctured  with  the  point  of  a 
pair  of  forceps. 

Several  types  of  injury  that  remove  the  style,  stigma  or  stamens 
before  pollination  may -cause  fall  by  preventing  fertilization.  It  is 
evident,  therefore,  that  fall  occurring  after  such  an  operation  per- 
formed on  the  flower  before  anthesis  may  be  due  to  lack  of  fertiliza- 
tion and  not  to  the  injury.  If,  however,  the  fall  occurs  within  the 
minimum  time  elapsing  between  anthesis  and  normal  flower-fall  due 


392 


University  of  California  Publications  in  Botany         [VOL.  5 


to  lack  of  fertilization,  it  can  be  safely  concluded  that  the  fall  is  due 
to  the  effect  of  the  injury.  This  minimum  time  is  about  seven  days 
for  N.  Langsdorffii.  It  can  be  safely  said,  therefore,  that  any  fall 
occurring  in  less  than  seven  days  after  injury  to  the  flower  near 
anthesis  is  due  directly  to  the  effect  of  the  injury.  In  cases  where  the 
stamens  or  style  are  removed  in  flowers  younger  than  those  at  anthesis, 

TABLE  2 

EFFECT  OF  DIFFERENT  TYPES  OF  INJURY  IN  CAUSING  FLOWER  FALL  IN 
N.  Langsdorffii  var.  ffrandi  flora 


No. 
flowers 

Size  or 
condition 
of  flowers 

Injury  to 

Avg.  No. 
days  before 
fall  of 
remaining 
organs 

Calyx 

Corolla 

Stamens 

Pistil 

Pedicel 

'10 

II-VII 

all  cut 

all  cut 

all  cut 

all  cut 

1 

10 

VII-XI 

<  i 

t  < 

t 

2 

H  - 

10 

XII-XIII 

a 

<  t 

t 

3 

.   2 

XIV 

1  1 

1  1 

' 

no  fall 

'   4 

I-II 

I  cut 

1  cut 

1  1 

style  cut 

7 

3 

III-VII 

(  < 

1  1 

' 

7 

3 

VIII-IX 

tl 

1  1 

i 

7 

}, 

5 

XI  -XII 

It 

1  1 

• 

no  fall 

u  - 

2 

XI-XII 

11 

<  t 

style  and 

5 

part  of 

ovary  cut 

.   3 

XIII-XIV 

1  1 

I  i 

it 

1  1 

4 

f   5 

V-VII 

i  cut 

1  1 

I  style  cut 

7 

oJ    5 

VIII-IX 

<  « 

it 

1  1 

6 

2 

XI 

<  I 

i  » 

tt      » 

9 

I    3 

XII 

t  i 

" 

1  1 

no  fall 

(   4 

I 

all  cut 

3 

&\    I 

II 

t  i 

no  fall 

112 

III-XII 

it 

1  1 

f    2 

V-VII 

slit  on  sides 

2  slit  on 

7 

to  base 

2  sides 

to  base 

1    8 

m-xi 

no  fall 

t?  " 

3 

II 

ovarv  slit 

3 

3 

V-VII 

1  1 

5 

3 

IX 

1  1 

2 

k   2 

XII 

1  1 

5 

'  5 

IX 

anthers 

10 

£ 

all  cut 

\     2 

V-VII 

all  cut 

7 

I   3 

VII  -X 

1  1 

9 

4 

VIII 

stigma  cut 

9 

g 

2 

V-VII 

style  cut 

7 

2 

VII-X 

i  i 

10 

f   4 

XIV 

all  cut 

all  cut 

no  fall 

2 

XIII 

i  cut 

i  cut 

2 

• 

2 

XIV 

n 

1  1 

no  fall 

4 

XIII-XIV 

slit  to  base 

slit  to  base 

« 

f  10 

II 

slit  to  base 

a 

3 

IX 

1  1 

n 

6 

I-VIII 

1  cut 

1  1 

1  H 

i  through 

8 

II-XII 

2  cuts 

1  1 

• 

i  through 

1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    393 


allowance  must  be  made  for  the  approximate  number  of  days  preced- 
ing anthesis.  Thus,  if  a  flower  of  the  above  species  is  injured  three 
days  before  anthesis,  the  fall  can  not  be  assigned  to  the  injury  unless 
it  occurs  before  ten  daj^s  have  elapsed.  The  minimum  time  for 
Fj  H179  is  about  five  days ;  thus,  any  time  of  five  days  or  more  recorded 
on  a  flower,  injured  near  anthesis,  was  considered  as  "no  fall."  The 
minimum  time  for  Lycopersicum  is  about  six  days. 

Finally,  it  is  necessary  to  state  that  the  process  of  reaction  to  the 
different  types  of  injury  recorded  in  the  following  tables  was  by  no 
means  impeded  by  low  temperatures.  Nicotiana  Langsdorffii  was 
tested  out  in  a  greenhouse  where  the  average  temperature  approxi- 
mated 75°  F.  The  tests  on  Fx  H179  and  Lycopersicum  were  per- 
formed in  the  botanical  garden  of  the  University  during  July  and 
August,  when  the  temperature  was  also  comparatively  high. 

The  following  statement  of  results  is  derived  in  great  part  but  not 
entirely  from  the  foregoing  tables.  It  has  been  noticed  that  cutting 
off  the  freshly  opened  flower  at  the  tip  of  the  pedicel  causes  the 
remainder  of  the  pedicel  to  be  thrown  off  in  from  ten  to  fifteen  hours, 
but  after  the  same  operation  on  developed  capsules  the  pedicel  re- 
mains firm  from  thirty-six  to  ninety-six  hours  after  the  injury. 
Removal  of  the  calyx  causes  the  fall  of  buds  in  two  or  three  days, 
depending  upon  the  age  of  the  bud.  Removal  of  half  the  calyx 
together  with  two-thirds  of  the  corolla  and  all  the  stamens  causes 
fall  in  one  to  four  days,  depending  upon  the  age  of  the  flower.  A 

TABLE  3 

EFFECT  OF  POLLINATION  OF  FLOWERS  OF  -N.  Langsdorffii  var.  grandiflora  ON 
REACTION  TO  INJURY 


No. 
flowers 

Pollination 

Injury 

Avg.  No. 
days  before 
fall 

41 

pollinated  when  injured 
not  pollinated 

calyx  and 

stamens  cut 

no  fall 
10 

,  /    4 

pollinated  when  injured 

calyx     " 

i  corolla  cut 

no  fall 

1    5 

not  pollinated 

tt 

8 

No.  days  after  pollination  when 

injured 

f  2 

1 

all  organs 

cut  at  tip  of  pedicel 

2 

1    2 

2-6 

tt 

2 

c  j    2 

7-8 

<  f 

2 

1    3 

2 

4    calyx, 

§    corolla,    stamens. 

4 

I 

style  cut 

f    3 

4-5 

11 

5 

dJ    2 

6-7 

(  i 

2 

dl     1 

9 

« 

3 

I    1 

9 

i  ( 

no  fall 

394 


•University  of  California  Publications  in  Botany         [V°L.  5 


transverse  cut  through  the  entire  flower  which  passes  through  the 
middle  of  the  ovary  causes  fall  in  one  to  two  days.  A  similar  oper- 
ation in  the  case  of  maturing  fruits  changes  the  date  of  fall  to 
four  to  eight  days.  Kemoval  of  half  the  corolla  and  all  the  stamens 
causes  fall  of  buds  in  one  day  and  the  fall  of  young  flowers  in  two  to 
three  days.  Removal  of  the  stamens  or  style  in  buds  causes  fall  in 

TABLE  4 
EFFECT  OF  DIFFERENT  TYPES  OF  INJURY  IN  CAUSING  FLOWER  FALL  IN  FJI179 


No. 

flowers 

Size  or 
condition 
of  flowers 

Injury  to 

Avg.  No. 
days  before 
fall  of 
remaining 
organs 

Calyx 

Corolla 

Stamens 

Pistil 

Pedicel 

/    9 

11-  VIII 

i  cut 

£  cut 

all  cut 

style  cut 

1 

a  \    6 

XI-XV 

<  t 

i  i 

t  t 

« 

no  fall 

{4 

III-VII 

i  cut 

1  1 

1 

10 

VIII-IX 

<  i 

11 

no  fall 

c     10 

v-vm 

I  I 

" 

r  9 

I 

all  cut 

2 

7 

II 

2 

3 

II 

no  fall 

4 

III-IV 

3 

• 

6 

III-IV 

no  fall 

1 

V 

2 

4 

V-VII 

no  fall 

2 

IX 

" 

'   7 

III-IV 

tt 

2 

1 

V 

4< 

5 

e- 

3 

V 

tt 

no  fall 

6 

VI  -VII 

a 

<  t 

{5 

II-VIII 

" 

2 

4 

VII  -VIII 

*< 

no  fall 

•    i 

II 

1  slit  on  2 

1  slit  on  2 

5 

sides  to 

sidos  to 

base 

base 

1 

II 

tt 

" 

no  fall 

9 

I  V-VII 

a 

(i 

tt 

§>  H 

9 

II 

2  slits  on  2 

2  slits  on  2 

1 

sides  to 

sides  to 

base 

base 

2 

II-IV 

st 

11 

4 

5 

V-VII 

11 

tt 

no  fall 

5 

II-V 

punctured 

punctured 

ovary 

2 

on  both 

on  both 

punctured, 

sides 

sides 

small  hole 

h- 

3 

VI  -VII 

" 

" 

<  i 

no  fall 

3 

VII  -XI 

tt 

t  < 

tt 

2 

3 

Il-III 

i  i 

tt 

tt 

2 

3 

VI-X 

« 

<  i 

" 

2 

15 

III-XII 

1  slit  to 

no  full 

base 

i- 

5 

punctur'd 

1 

many 

l 

times 

f   6 

XIV 

i  cut 

capsule 

4 

j  J 

i  cut 

I   3 

XIV 

" 

<  t 

no  fall 

1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    395 

two  to  four  days.    Severe  injury  of  any  kind  to  the  ovary  causes  fall 
in  one  to  two  days. 

The  figures  given  above  for  the  reaction  time  in  cases  of  abscission 
following  mechanical  injury,  together  with  a  more  detailed  considera- 
tion of  the  tables,  indicate  that  the  reaction  time,  in  general,  does  not 
depend  so  much  on  the  type  of  injury  as  on  the  age  of  the  flower 
concerned.  What  connection  there  is  between  the  type  of  injury  and 
the  reaction  time  seems  to  be  based,  except  in  cases  of  injury  to  the 
ovary,  on  the  relation  of  the  amount  of  material  removed  to  the 
amount  remaining.  Thus,  cutting  off  the  flower  at  the  tip  of  the 
pedicel  causes  abscission  of  the  remaining  pedicel  more  quickly  than 
any  other  type  of  injury.  One  exception  to  this  statement  is  seen,  as 


TABLE  5 

EFFECT  OF  DIFFERENT  TYPES  OF  INJURY  IN  CAUSING  FLOWER  FALL  IN 
Lycopersicum  esculentum 


No. 
flowers 

Size  or 
condition 
of  flowers 

Injury  to 

Avg.  No. 
days  before 
fall  of 
remaining 
organs 

Calyx 

Corolla 

Stamens 

Pistil 

Pedicel 

a- 

'   4 
4 

I 
II  -VIII 

all  cut 

i  I 

no  fall 

a 

.   6 

XII 

(  < 

14 

r   3 

XII 

entire 

2 

ovary  cut 

3 

XII1-XIV 

ovary 

no  fall 

punctured 

4  times  on 

!>- 

top 

1 

XII 

<  < 

3 

4 

XII 

ovary 

2 

punctured 

4  times  on 

side 

,   3 

XIV 

it 

no  fall 

'   4 

II 

punctured 

punctured 

ovary 

9 

. 

at  base 

punctured 

c- 

once  on 

side 

L   4 

VIII 

(i 

1  1 

1  1 

4 

r  4 

II-  VIII 

i  cut 

i  cut 

no  fall 

4 

VIII  -IX 

<  < 

<  4 

tt 

5 

d 

3 

I-II 

i  < 

ovary  i  cut 

1 

3 

VIII 

1  1 

ft 

« 

3 

I    2 

IX 

<  t 

i,  I 

<  < 

2 

e 

5 

I-IX 

i  i 

all  cut 

no  fall 

f 

r  s 

L    6 

VIII 
X-XI 

style  cut 

a 

5 

no  fall 

g 

5 

VIII  -XIV 

slit 

<  i 

r  3 

VIII 

all  cut 

<  i 

4 

h 

5 

t  ( 

1  1 

no  fall 

L   4 

II-VIII 

a 

<  < 

396  University  of  California  Publications  in  Botany         [VOL.  5 

indicated  above,  in  the  case  of  injury  to  the  ovary  in  which  this  organ 
may  be  merely  punctured,  without  necessarily  removing  any  material, 
yet  abscission  occurs  in  one  to  two  days  after  the  injury. 

It  has,  on  the  other  hand,  been  evident  throughout  all  the  abscis- 
sion experiments  that  age  of  flower  is  the  important  factor  in  deter- 
mining the  reaction  time,  older  flowers  nearly  always  responding  more 
slowly  to  stimulation  by  injury  than  younger  ones.  It  will  be  seen, 
however,  from  the  tables  that  there  are  occasionally  individual  excep- 
tions to  the  general  rule.  These  exceptions  might  be  explained  in  a 
number  of  ways.  For  example,  it  is  possible  in  the  case  of  older  flow- 
ers that  the  ovary,  having  increased  in  size,  was  accidentally  cut  in 
the  operation  of  injury,  thus  adding  the  extra  factor  of  stimulation 
of  the  ovary  which  in  younger  flowrers  would  not  be  present.  In  gen- 
eral, such  exceptions  to  the  general  rule  indicate  to  what  extent  the 
normal  or  abnormal  physiological  conditions  of  the  plant  enter  into 
the  problem. 

2.  ABSCISSION  TIME 

The  abscission  time,  or  the  actual  time  involved  in  the  process  of 
cell  separation,  was  considered  in  a  preliminary  paper  (Goodspeed  and 
Kendall,  1916)  wherein  the  minimum  time  in  which  abscission  was 
known  to  have  occurred  was  stated  to  be  from  four  to  eight  hours  in 
normal  abscission  and  from  one  to  four  hours  in  "spontaneous" 
abscission.  A  few  additional  data  are  now  at  hand  in  the  case  of 
Fx  H179  and  Nicotiana  Tabacum  ' '  Maryland. ' '  These  two  forms, 
as  has  already  been  noted,  are  a  little  more  sensitive  than  most 
Xicotiana  varieties  and  normal  abscission  was  found  to  take  place  in 
from  three  to  six  hours. 

The  time  of  cell  separation  in  "spontaneous"  abscission  can  be 
more  exactly  determined  than  that  in  normal  abscission  because  of 
the  regularity  with  which  the  plants  respond  to  certain  conditions  of 
injury  or  to  the  presence  of  narcotic  vapors.  Data  on  this  point  were 
obtained  in  the  following  manner.  Flowering  shoots  with  flowers  of 
different  sizes  were  cut,  placed  in  water  and  inserted  under  a  bell-jar. 
Enough  illuminating  gas  was  then  introduced  under  the  jar  to  make 
1.5  per  cent  approximately.  The  temperature  during  the  experiment 
was  practically  constant  at  19°  C.  After  the  shoot  had  been  left  in 
this  abnormal  atmosphere  for  five  hours  a  few  flowers  were  picked  off 
at  fifteen-minute  intervals  and  free-hand  sections  made  of  their 
pedicels  until  flowers  about  the  size  of  those  which  were  being  sec- 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solan-aceae    397 

tioned  began  to  fall.  It  was  found  that  signs  of  abscission  hardly 
ever  appeared  until  thirty  to  forty  minutes  before  actual  fall  occurred. 
This  indicates  that  the  actual  process  of  cell  separation  in  F!  H179 
takes  place  in  from  thirty  to  forty  minutes.  Experiments  carried  on 
in  the  same  manner  with  N.  Tabacum  "Maryland"  indicate  that 
abscission  here  takes  place  in  from  forty-five  to  sixty  minutes. 

Both  the  reaction  time  of  abscission  and  the  actual  abscission  time 
are  profoundly  influenced  by  temperature  and  by  humidity.  Varia- 
tion in  the  intensity  of  the  illumination,  however,  seems  to  have  no 
direct  influence  upon  abscission.  In  comparing  the  effect  of  changes 
in  temperature  and  humidity  it  was  found  that  the  results  of  experi- 
ments intended  to  show  the  time  of  abscission  are  far  more  dependent 
upon  temperature  than  upon  humidity.  This  is  not  because  changes 
in  humidity  have  little  influence  upon  abscission  but  because  such 
changes  have  to  be  very  great  indeed  before  bringing  about  any  appre- 
ciable effect.  Very  slight  changes  in  temperature,  on  the  other  hand, 
often  influence  abscission  to  a  marked  degree.  Abscission  goes  on 
very  actively  under  high  temperatures  and  converse^  very  slowly 
under  low  temperatures.  It  starts  in  the  case  of  Fx  H179  about  seven 
hours  after  insertion  in  1.5  per  cent  illuminating  gas  at  a  temperature 
of  19°  C.  If  the  same  experiment  be  repeated  in  a  temperature  of 
approximately  9°  C.  abscission  may  not  occur  for  fifteen  to  twenty- 
four  hours. 

Drought  has  to  be  quite  severe  before  retarding  abscission.  There 
is  110  doubt,  however,  that  wilted  shoots  will  not  drop  flowers  as 
quickly  as  fresh  ones  and  if  the  wilting  proceeds  far  enough  no  abscis- 
sion will  occur.  This  effect  is  all  the  more  noticeable  if  the  air  around 
the  wilted  shoot  is  kept  free  from  moisture. 


EXPERIMENTAL  INDUCTION  OF  ABSCISSION 
1.  INDUCTION  BY  ILLUMINATING  GAS 

The  first  subject  to  be  considered  under  this  heading  is  the  com- 
parative effect  of  illuminating  gas  in  causing  abscission  in  several 
species  of  the  Solanaceae.  The  method  of  determining  this  consisted 
largely  in  placing  flowering  shoots  of  the  different  species  in  water 
under  bell-jars  and  introducing  enough  illuminating  gas  under  the 
jars  to  make  the  percentage  of  narcotic  vapors  in  the  air  around  the 
plant  1.5.  The  temperature  during  the  experiments  was  compara- 


398  University  of  California  Publications  in  Botany         [VOL.  5 

tively  high,  ranging  from  15°  to  20°  C.  The  results,  which  were 
recorded  approximately  fifteen  hours  after  subjection  to  the  gas,  arc 
given  in  the  following  table : 

TABLE  6 

Species,   variety,    or  Amount  of  abscission,  expressed  almost  entirely 

hybrid  in  terms  of  size  of  flowers  thrown  off 

N.  Tabacum  var.  macrophylla all  buds  up  to  anthesis. 

N.  Tabacum  " Maryland" all  flowers  up  to  4  or  5  days  past  anthesis. 

F,  H154  all  buds  up  to  opening  of  corolla. 

F,  H36  all  buds  and  flowers. 

F,H179  all  buds  and  flowers. 

N.  glauca  young  buds. 

N.  rustica  var.?  buds  up  to  anthesis. 

N.  rustica  var.?  buds,  flowers,  and  fruits. 

N.  Bigelovli  var.  Wallacei no  abscission. 

N.  Bigelovii  "Porno"   no  abscission. 

N.  quadrivalvis no  abscission. 

N.  multivalvis    no  abscission. 

N.  Sanderae    buds  up  to  anthesis. 

N.  suaveolens   buds  up  to  anthesis. 

N.  plumbaginifolia    buds  up  to  opening  of  the  corolla. 

Solanum  umbelliferum  ^  small  buds. 

S.  jasminioides buds  and  flowers. 

S.  verbascifolium  no  abscission. 

S.  nigrum  small  buds. 

lochroma  tuberosa  no  abscission. 

Oestrum  fasciculatum  buds  and  flowers. 

Lycopersicum  esculentum  var. 

pyriforme  no  abscission. 

L.  esculentum  var.  vulgare  small  buds  and  occasional  flowers. 

Petunia  hybrida no  abscission. 

Salpiglossis    sinuata    -no  abscission. 

Datura  sanguineum  buds  and  flowers. 

Salpichrora  rhomboidea no  abscission. 

Lycium  australis   -  no  abscission. 

As  might  be  expected,  most  of  these  varieties  react  to  laboratory 
air  in  the  same  manner  that  they  do  to  illuminating  gas.  In  the  case 
of  laboratory  air  a  longer  time  and  higher  temperature  is  generally 
required  before  the  reaction  occurs.  All  the  species,  with  the  excep- 
tion of  those  which  throw  off  only  young  buds,  detach  most  of  their 
flowers  when  left  in  laboratory  air  overnight.  If  a  window  or  two  is 
left  open,  allowing  fresh  aid  to  enter  and  at  the  same  time  lowering 
the  temperature,  no  abscission  occurs. 

It  was  found  that  several  of  the  species  recorded  above,  in  which 
no  abscission  or  very  little  abscission  occurred,  detached  more  flowers 
when  a  larger  percentage  of  gas  was  used  or  when  subjected  to  1.5 
per  cent  gas  for  a  longer  time.  Thus,  both  varieties  of  Lycopersicum 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    399 

esculent um,  lochroma  tuberosa,  Solanum  nigrum,  and  8.  verbasci- 
folium,  upon  subjection  to  3  per  cent  illuminating  gas  for  twenty 
hours,  throw  off  all  flowers  up  to  those  two  or  three  days  past  anthesis. 
No  abscission  occurred,  however,  in  any  concentration  of  gas,  in 
Mcotiana  Bigelovii,  N.  quadrivalvis,  N.  multivalvis,  Lycium  australis, 
Petunia  hybrida,  Salpiglossis  stinuata,  or  Salpichrora  rhomlioidea. 

A  peculiar  condition  exists  in  Solanum  umbelliferum,  which  throws 
off  buds  in  the  illuminating  gas  but  never  under  any  conditions,  in- 
cluding temperature  or  the  presence  of  narcotic  vapors,  throws  off 
flowers  in  which  the  corolla  has  fully  opened.  A  corresponding  con- 
dition seems  to  exist  in  Nicotiana  Tabacum  var.  macrophylla,  FJH154, 
N.  Sander ae,  N.  rustica  var.  brasUia,  and  in  one  other  variety  of  N. 
rustica,  all  of  which  seldom  under  any  conditions  detach  fully  opened 
flowers,  although  flowers  up  to  that  stage  are  freely  abscissed.  Thus 
there  seems  to  be,  in  certain  species  and  at  about  the  time  of  the  open- 
ing of  the  corolla,  a  sudden  increase  in  resistance  to  the  external 
stimulus  which  is  causing  abscission.  In  other  species  this  sudden 
increase  in  resistance  does  not  take  place,  abscission  commonly  occur- 
ring at  any  stage  in  the  development  of  the  flower  or  fruit  and  the 
increase  in  resistance  taking  place  very  gradually.  In  addition,  there 
seems  to  be  an  intergradation  of  forms  between  those  in  which  the 
increase  in  resistance  takes  place  suddenly  and  those  in  which  it  takes 
place  gradually. 

The  next  subject  to  be  taken  up  is  a  consideration  of  experiments 
5,  6,  7,  8,  and  9  on  the  induction  of  abscission  in  small  isolated  pieces 
of  the  pedicel.  The  main  purpose  of  devising  these  experiments  was  to 
throw  some  light,  if  possible,  on  the  direct  or  indirect  action  of  the 
external  factor  in  causing  "spontaneous"  abscission.  The  pedicel  of 
Fx  H179  was  again  chosen  as  material  for  the  following  experiments, 


Fig.  9 


400 


University  of  California  Publications  in  Botany         [VOL.  5 


largely  because  of  the  ease  and  regularity  with  which  abscission  is 
induced  in  this  hybrid  by  sudden  changes  in  the  external  environment. 

Experiment  5. — This  experiment  was  devised  to  discover  the  effect 
of  reducing  the  volume  of  material  proximal  to  the  separation  layer 
on  the  abscission  of  flowers  of  Nicotiana  as  induced  by  illuminating 
gas.  Two  series  of  flowers  were  cut  as  in  figure  9.  In  the  last  two 
flowers  represented  on  the  right  the  cut  was  made  less  than  0.5  mm. 
from  the  separation  layer.  These  flowers  were  then  rolled  in  damp 
filter  paper  and  left  in  1.5  per  cent  illuminating  gas  overnight.  After 
fifteen  hours,  abscission  had  occurred  in  all  the  flowers  except  the  one 
represented  on  the  extreme  right  in  the  figure.  Abscission  had 
occurred  in  one  flower  in  which  the  cut  had  been  made  less  than 
0.5  mm.  from  the  separation  layer.  The  control  to  this  experiment 
showed  that  abscission  does  not  occur  for  several  days  in  a  series  of 
flowers  cut  as  in  figure  9  and  kept  under  normal  conditions. 

Experiment  6. — This  experiment  was  devised  to  show  the  effect 
upon  abscission  of  reducing  the  volume  of  material  distal  as  well  as 
proximal  to  the  separation  layer.  In  this  case  the  flowers  were  cut  off 
at  varying  distances  from  the  separation  layer,  making  the  series 
shown  in  figure  10.  The  last  two  pieces  on-  the  right  in  this  series 
were  cut  less  than  0.5  mm.  on  each  side  of  the  separation  layer  so 
that  the  total  length  of  the  pieces  was  not  much  above  1  mm.  In  this 
experiment  and  in  similar  ones  which  follow  it  was  necessary  to  keep 


Fig.  10 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    401 

the  material  moist.  This  was  accomplished  in  various  ways,  but  the 
best  method  was  found  to  consist  in  placing  the  pieces  on  a  long  strip 
of  filter  paper  one  end  of  which  rested  in  water.  In  this  experiment 
abscission  occurred  after  ten  hours  subjection  to  1.5  per  cent  illum- 
inating gas  in  all  except  the  two  pieces  represented  in  the  extreme 
right  of  figure  10.  Abscission  here  took  place  in  several  pieces  rang- 
ing from  1  mm.  to  2  mm.  in  length.  A  microscopic  examinatiDimf  the 
separation  surfaces  indicated  that  the  process  of  abscission  corre- 
sponded entirely  with  normal  abscission  as  it  occurs  in  plants  in  the 
field.  Experiments  made  in  a  similar  manner  upon  N.  Tabacum 
"Maryland"  and  Lycopersicum  gave  similar  results.  In  the  control, 
which  consisted  in  keeping  pieces  of  the  pedicel  as  shown  in  figure.  10 
under  normal  atmospheric  conditions,  abscission  occurred  after  about 
twenty  hours,  evidently  as  the  result  of  no  other  stimulus  than  that 
caused  through  cutting  oft3  the  flower  by  severing  the  pedicel.  The 
reaction  in  the  control,  however,  is  much  slower  than  in  the  case  in 
which  the  added  effect  of  the  illuminating  gas  is  operative,  indicating 
that  the  latter  factor,  although  it  here  serves  merely  to  hasten  the 
abscission  process,  has  an  effect  of  some  kind  on  the  tissues  at  the 
base  of  the  pedicel. 

Following  these  two  experiments,  a  number  of  attempts  were  made 
in  the  same  way  to  induce  abscission  in  longitudinal  free-hand  sections 
of  the  pedicel  cut  for  microscopical  examination.  It  was  soon  discovered 
that  the  abscission  process  could  be  induced  in  the  separation  zone  in 
thick  longitudinal  sections  of  the  pedicel  by  subjecting  them  to  high 
percentage  (5  to  7  per  cent)  of  illuminating  gas.  Cell  separation  in 
cross-sections  through  the  separation  zone  could  not  be  induced  by  any 
means  at  hand.  The  following  experiments  give  more  detailed  results 
in  this  connection. 

Experiment  7. — In  this  experiment,  median,  longitudinal  sections 
of  varying  thickness  were  cut  through  the  pedicels  so  that  the  plane 
of  the  sections  corresponded  with  the  plane  formed  by  both  the 
pedicel  and  the  main  axis  of  the  inflorescence.  These  sections  were 
subjected  to  7  per  cent  illuminating  gas,  care  being  taken  to  keep  them 
moist,  but  not  submerged,  throughout  the  entire  experiment.  The 
best  arrangement  was  found  to  be  one  in  which  the  sections  rested  in 
a  thin  film  of  water  on  one  side  but  were  exposed  to  the  air  on  the 
other.  After  several  hours  in  the  7  per  cent  illuminating  gas,  abscis- 
sion started  in  the  thicker  sections  but  not  in  the  thinner  ones.  The 
extent  to  which  abscission  proceeded  depended  upon  the  thickness  of 


402  University  of  California  Publications  in  Botany         [VOL.  5 

the  section.  Abscission  became  complete  in  sections  0.3  mm.  or  more 
in  thickness,  the  separation  taking  place  in  such  a  way  that  a  slight 
bending  or  pulling  motion  sufficient  to  break  the  trachea  divided  the 
section  into  equal  halves.  In  thinner  sections,  ranging  from  0.3  mm. 
to  0.17  mm.,  abscission  starts  in  the  normal  position  but  does  not  pro- 
ceed to  completion,  the  extent  to  which  the  process  takes  place  depend- 
ing, as  has  been  said,  upon  the  thickness  of  the  section.  In  sections 
much  below  0.17  mm.  no  signs  of  abscission  appear.  Also,  if  the 
thicker  sections  are  shortened  in  length  to  any  considerable  extent  by 
cutting  off  portions  of  the  tissues  from  either  side  of  the  separation 
layer,  abscission  will  not  occur. 

The  process  of  abscission  as  it  occurs  in  these  sections  corresponds 
exactly  to  the  process  in  an  entire  pedicel.  Cell  separation  starts 
independently  in  the  pith  and  in  the  cortex,  appearing  first  in  that 
part  of  the  cortex  corresponding  to  the  ventral  region  of  the  pedicel 
where,  it  will  be  remembered,  abscission  starts  in  the  entire  flower. 
When  mounting  the  sections  on  an  object  slide  for  microscopical 
examination,  the  isolated  cells  in  the,  pith  lie  in  position  but  can  be 
easily  washed  out  with  a  small  jet  of  water.  In  the  cortex  a  break 
soon  appears  in  the  epidermis  as  the  result  of  manipulation  in  mount- 
ing and  a  cavity  is  formed  at  that  point  as  the  result  of  the  isolated 
cells  of  the  cortex  floating  out  in  the  water. 

Experiment  7  was  repeated  in  the  case  of  Datura  with  similar 
results,  except  that  in  this  case  abscission  was  more  active  since  it 
involved  more  cells,  a  situation  which  one  might  be  led  to  expect 
because  of  the  differences  between  the  two  species  in  the  normal  abscis- 
sion of  entire  flowers.  It  will  be  remembered  that  the  separation  cells 
of  the  cortex  in  Datura  are  in  no  way  distinguishable  from  other 
cortical  cells ;  yet  even  in  these  sections  separation  occurs  in  a  definitely 
predetermined  position  corresponding  entirely  with  the  position  in 
abscission  of  the  entire  flower.  It  was  even  noticed  that  abscission 
started  in  these  sections  in  the  same  tissues  and  in  the  same  manner 
as  in  normal  floral  abscission. 

After  the  thickness  of  the  sections  best  adapted  to  obtaining  results 
had  been  determined,  the  following  experiment  was  performed  on 
sections  cut  from  different  parts  of  the  pedicel. 

Experiment  8. — In  this  experiment  a  series  of  longitudinal  sections 
of  the  pedicel  were  cut  so  that  the  plane  of  the  sections  was  at  right 
angles  to  that  of  the  sections  cut  in  Experiment  7.  The  first  section 
was  tangential,  on  the  ventral  side  of  the  pedicel,  and  contained  only 
the  epidermis  and  a  few  tiers  of  cortical  cells.  Section  2  was  also 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    403 

tangential  but  contained  a  few  tracheae  on  one  surface.  Section  3  was 
more  or  less  radial,  containing  two  strands  of  vascular  tissue  on  either 
side.  Sections  4  and  5  were  similar  to  sections  1  and  2.  On  subject- 
ing these  sections  to  illuminating  gas  it  was  noticed  that  abscission 
started  first  in  sections  1,  2,  and  3,  appearing  last  in  sections  4  and  5. 
This  result  is  exactly  parallel  with  the  process  as  it  occurs  in  normal 
abscission,  where  the  process  starts  first  in  the  ventral  cortex"  and  in 
the  pith. 

In  passing,  mention  might  be  made  of  the  peculiar  reaction  of  the 
tangential  sections  2  and  4,  which  were  made  up  almost  entirely  of 
cortical  cells  with  a  few  vascular  elements  on  one  side.  When  abscis- 
sion occurred  in  these  sections,  a  bending  or  bowing  of  the  section  was 
alwa.ys  noticed.  This  bending  was  always  such  that  the  tracheal  tissue 
was  on  the  concave  side,  as  if  the  cells  of  the  cortex  had  undergone 
considerable  expansion  while  the  cells  of  the  vascular  tissue  retained 
their  original  size.  From  the  work  of  Richter  and  others,  it  may  be 
expected  that  subjection  of  portions  of  plant  tissues  to  illuminating 
gas  would  cause  an  increase  in  turgor  in  the  cells  concerned.  Thus,  it 
is  probable  that  the  bending  of  the  sections,  as  described  above,  is  due 
to  the  increase  in  turgor  of  the  cortical  cells  caused  by  the  narcotic 
effect  of  the  illuminating  gas.  The  extent  of  the  bending  was  such 
that  most  of  the  cells  in  the  cortex  as  well  as  the  separation  cells  must 
have  been  involved  in  the  process.  On  repeating  the  above  experiment 
with  Datura,  a  similar  bending  of  the  tangential  sections  was  even 
more  pronounced  than  in  Nicotiana. 

Experiment  9. — As  mentioned  above,  efforts  to  induce  abscission 
failed  in  thin  sections.  The  sections  in  Experiment  9  were  cut  so  that 
they  were  thin  in  the  separation  layer  but  thick  on  either  side.  Both 
surfaces  of  these  sections  were  thus  cut  slightly  concave  so  that  the 
sections  were  thickest  at  the  ends  and  thinnest  in  the  middle,  where 
the  separation  zone  was  located.  The  sections  were  then  subjected  to 
7  per  cent  illuminating  gas  as  in  Experiment  7.  It  was  not  possible 
to  cut  very  thin  free-hand  sections  of  the  shape  described,  but  it  was 
demonstrated  without  a  doubt  that  abscission  occurred  in  sections  of 
this  peculiar  shape  which  were  thinner  in  the  separation  zone  than 
thoso  in  Experiment  7  where  abscission  had  failed  to  occur. 

"  Certain  conclusions  which  can  be  drawn  from  experiments  5,  6,  7, 
8,  and  9  are  given  below. 

1.  Abscission  can  be  induced  by  allowing  the  external  factor  to  act 
directly  upon  the  cells  in  the  vicinity  of  the  separation  zone  (Expts. 
6,  7,  and  8). 


404  University  of  California  fiiblications  in  Botany         [VOL.  5 

2.  Abscission  induced  by  the  above  methods  in  isolated  pieces  must 
be  independent  of  transportation  of  material  from  the  rest  of  the  plant. 

3.  The  fact  that  abscission  cannot  be  induced  in  thick  cross-sections 
of  the -separation  zone  shows  that  cell  separation  cannot  be  induced 
by  the  action  of  the  external  factor  directly  on  the  separation  cells. 

4.  It  is  necessary  that  a  certain  proportion  of  the  tissues  of  the 
pedicel  be  in  intercellular  connection  with  the  cells  of  the  separation 
zone  before  cell  separation  will  occur,  but  this  proportion  is  surpris- 
ingly small  (Expts.  7,  8,  and  9). 

5.  There  is  evidently  increase  in  turgor  in  all  the  cortical  cells  of 
the  pedicel  during  abscission  induced  by  the  above  method  (Expt.  8). 


2.  ACTION  OP  ACIDS  ON  THE  SEPARATION  CELLS  OP  Nicotiana 

Under  this  heading  a  description  will  be  given  of  the  effect  of 
mineral  acids  on  small  isolated  pieces  such  as  were  used  in  experiments 
6,  7,  8,  and  9.  It  was  stated  above  (page  364)  that  by  the  use  of  two 
mineral  acids  together  with  several  stains,  no  chemical  difference  could 
be  detected  between  the  cell  walls  of  the  separation  cells  and  those  of 
normal  cortical  cells.  The  present  work  represents  an  attempt  to 
determine,  by  experimental  means  and  by  watching  through  the  micro- 
scope the  action  of  acids  on  cell  walls,  whether  the  cell  membranes 
of  the  separation  cells  are  more  subject  to  hydrolysis  than  those  of 
normal  cortical  cells. 

Experiment  10. — Small  pieces  of  the  pedicel  were  prepared  as  in 
figure  10.  These  pieces  were  boiled  for  one  or  two  minutes  in  4  per 
cent  hydrochloric  acid  and  then  washed  in  water.  Upon  examination 
it  was  found  that  the  pieces  could  be  separated  into  halves  through 
the  separation  zone  by  a  slight  pulling  or  bending  motion.  Microscopic 
examination  of  the  separation  surfaces  showed  that  the  break  through 
the  cells  of  the  separation  zone  had  taken  place  along  the  plane  of  the 
middle  lamellae  of  their  walls.  This  same  type  of  separation  was 
brought  about  without  boiling  when  10  per  cent  nitric  or  hydrochloric 
acid  was  allowed  to  act  on  the  pedicels  for  approximately  five  minutes. 
When  longitudinal  sections  are  used  in  place  of  entire  pedicles,  the 
same  results  are  obtained  but  much  more  rapidly.  It  was  also  noticed 
that  separation  under  these  latter  conditions  takes  place  more  quickly 
in  younger  pedicels  than  in  older  ones.  In  the  pedicels  of  fully 
developed  fruits  no  separation  could  be  induced,  but  in  those  of 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    405 

immature  fruits  separation  occurred  in  the  cortex  but  failed  to  take 
place  within  the  vascular  cylinder. 

Experiment  10  at  first  glance  would  seem  to  indicate  that  the  cell 
walls  of  the  separation  cells  are  more  subject  to  hydrolysis  than  normal 
cortical  cells.  Another  interpretation  is  possible,  however.  Actual 
separation  which  takes  place  through  the  separation  zone  may  be  due 
to  the  fact  that  the  cells  in  this  zone  are  small  and  have  "a  tendency 
to  be  isodiametric,  whereas  the  remaining  cells  of  the  cortex  are  larger 
and  are  elongated  parallel  to  the  long  axis  of  the  pedicel.  Hydrolysis 
of  the  cell  walls  may  go  on  with  equal  rapidity  in  all  the  cortical  cells 
at  the  base  of  the  pedicel,  yet  upon  bending  or  pulling  separation  may 
take  place  through  the  region  of  isodiametric  cells  because  of  the  inter- 
locking of  the  elongated  cells  in  the  rest  of  the  cortex.  An  attempt 
was  made  to  gain  further  evidence  on  this'  point  by  observing  through 
the  microscope  the  action  of  acids  on  the  cell  walls  of  the  tissues  con- 
cerned. "When  the  action  of  the  acids  is  thus  observed,  the  walls  are 
seen  to  soften  and  to  swell  to  two  or  three  times  their  normal  thick- 
ness. This  effect  is  all  the  more  noticeable  if  the  walls  initially  are 
comparatively  thick.  Now,  since  the  cells  of  the  separation  zone  are 
small  and  somewhat  collenchymatous,  or  at  least  have  thicker  walls 
than  normal  cortical  cells,  the  process  of  swelling  in  the  cell  wall  is 
most  conspicuous  in  that  region.  Indeed,  hardly  any  swelling  can  be 
perceived  as  a  result  of  the  acid  treatment  in  the  cell  walls  of  normal 
parenchyma  cells  of  the  cortex.  However,  when  a  form  such  as 
Lycopersicum  is  examined  in  which  there  is  a  distinct  layer  of  col- 
lenchyma  beneath  the  epidermis  for  the  entire  length  of  the  pedicel, 
this  collenchyma  appears  to  be  affected  at  the  same  time  and  in  the 
same  manner  as  the  cells  of  the  separation  zone  of  Nicotiana.  Also 
in  Nicotiana  there  seems  to  be  a  certain  amount  of  similarity  in 
reaction  to  acids  between  the  smaller  cells  of  the  cortex  just  beneath 
the  epidermis  and  those  of  the  separation  zone.  The  conclusion  can 
thus  be  drawn  that  the  cell  walls  of  the  separation  cells  are  no  more 
readily  hydrolyzed  than  those  of  normal  collenchymatous  tissues.  Of 
course,  the  fact  still  remains  that  the  collenchyma  of  the  cortex  may 
be  more  subject  to  hydrolysis  than  the  cortical  parenchyma.  Now 
the  small  cells  of  the  separation  zone  not  only  extend  across  the  base 
of  the  pedicel  but  also  spread  throughout  the  general  region  at  the 
base  of  that  organ;  it  was  therefore  noticed  that  the  swelling  of  cell 
walls  was  by  no  means  confined  to  cells  of  the  separation  layer  but 
was  more  or  less  prominent  throughout  the  wyhole  general  region  at 
the  base  of  the  pedicel. 


406  University  of  California  Publications  in  Botany         [VOL.  5 

The  general  results  of  these  observations  are  in  a  sense  negative 
and  seem  to  indicate  that  the  walls  of  the  separation  cells  are  no  more 
subject  to  hydrolysis  than  the  walls  on  either  side.  This,  of  course, 
does  not  preclude  the  possibility  that  a  difference  exists  which  is  too 
slight  to  be  detected.  It  appears,  however,  that  the  general  region 
at  the  base  of  the  pedicel  may  be  more  subject  to  hydrolysis  than  the 
more  distant  portions. 

3.  INDUCTION  BY  MECHANICAL  INJURY 

The  results  of  experiments  on  the  induction  of  abscission  by  mechan- 
ical injury  are  recorded  in  tables  2,  3,  4,  and  5,  which  have  already 
been  considered  under  the  heading,  "Time  of  Abscission"  (page  384). 
Several  facts  of  interest  brought  out  by  table  2,  which  deals  with 
Nicotiana  Langsdorffii  var.  granctiflora,  are  summarized  below. 

1.  It  appears  that  removal  of  or  injury  to  the  capsule  does  not 
cause  abscission  in  mature  fruits  (table  2,  a,  5,  and  h;  table  3,  c  and 
d).     The  same  types  of  injury  generally  do  cause  abscission  in  im- 
mature fruits. 

2.  It  seems  that  a  transverse  cut  completely  through  the  flower  at 
the  distal  end  of  the  calyx  causes  abscission  only  in  buds  or  flowers 
near   anthesis    (table   2,   c).     It   appears,   however,   that   such   a   cut 
proximal  to  the  distal  end  of  the  calyx  causes  abscission  in  flowers 
several  days  past  anthesis  as  well  as  in  buds  (table  2,  a,  &). 

3.  Removal  of  the  entire  calyx  causes  fall  in  very  young  buds  only 
(table  2,  d). 

4.  It  seems  that  slitting  both  the  corolla  and  calyx  longitudinally 
on  both  sides  from  tip  to  base  does  not  induce  abscission  even  in  young 
buds  (table  2,  e). 

5.  Entire  removal  of  the  style  or  stamens  causes  fall  only  in  young 
buds  (table  2,  /  and  g). 

6.  It  appears  that  injuries  to  the  pedicel  do  not  cause  abscission, 
provided  the  flower  is  not  entirely  cut  away  (table  2,  i).    Just  here  it 
is  worth  mentioning  that  two  of  the   pedicels   cut  .transversely   as 
recorded  in  table  2,  i,  were  cut  so  deep  that  the  flowers  bent  over 
and  hung  only  by  a  few  vascular  strands  and  cortical  cells.      The 
wound  healed  over,  however,  and  the  two  flowers  matured  with  the  rest. 

7.  It  is  evident  that  injuries  which  reach  the  ovary  are  much  more 
effective  in  causing  abscission  than  injuries  affecting  the  other  parts 
of  the  flower  (table  2,  b  and  c). 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    407 

8.  Fertilization  has  no  influence  whatever  in  preventing  abscission 
when  the  latter  is  induced  by  a  transverse  cut  completely  through  the 
flower  at  the  base  or  middle  of  the  calyx  (table  3,  c  and  d). 

9.  Certain  types  of  injury,  such  as  entire  removal  of  the  calyx  and 
stamens  or  removal  of  the  entire  calyx  and  half  the  corolla,  evidently 
cause  abscission  only  by  preventing  fertilization  (table  3,  a  and  &). 

Taking  up  now  the  results  given  in  table  4,  which  "dealt  with 
Fj  H179,  it  will  be  seen  that  this  hybrid  is  more  sensitive  to  injury 
than  is  N.  Langsdorffli.  Nevertheless,  it  is  very  plain  that  the  general 
conclusions  announced  above  for  this  latter  species  hold  for  Ft  H179 
also.  There  follows  a  partial  summary  of  the  results  in  table  4  and 
a  comparison  of  these  results  with  those  obtained  in  the  experiments 
on  N.  Langsdorffii. 

1.  It  seems  that  removal  of  the  calyx  causes  fall  of  much  larger 
buds  than  in  N.  Langsdorffli  (table  4,  d). 

2.  F!  H179  is  evidently  much  more  sensitive  in  its  abscission  re- 
action to  a  transverse  cut  through  the  flower  at  the  middle  of  the  calyx 
than  N.  Langsdorffii  (table  4,  a). 

3.  It  would  seem  that  slitting  the  calyx  and  corolla  even  to  the 
extent  of  dividing  these  organs  into  four  longitudinal  strips  does  not, 
as  a  general  rule,  cause  abscission.     Such  an  injury  does  cause  abscis- 
sion only  in  extremely  small  buds  (table  4,  gr). 

4.  It   appears  that   puncturing  the   calyx,   corolla   and   ovary  so 
that  a  hole  is  formed  about  2  mm.  in  diameter  in  the  latter  organ 
causes  fall  in  flowers  of  all  sizes  up  to  two  or  three  days  past  anthesis 
(table  4,  7i).     Since  it  is  evident  that  such  a  hole  through  the  calyx 
and  corolla  alone  would  not  cause  abscission  (table  4,  #),  abscission 
in  this  case  must  be  induced  by  injury  to  the  ovary. 

5.  It  is  evident  that  a  slit  completely  through  the  pedicel  for  its 
entire  length  fails  to  cause  fall  in  buds  or  open  flowers,  but  where  an 
effort  is  made  to  destroy  completely  the  connection  between  the  flower 
and  stem  abscission  will  occur  (table  4,  i). 

6.  Removal  of  the  style  or  stamens,  as  a  general  rule,  causes  fall 
only  in  young  buds,  but  removal  of  the  former  organ  is  probably  more 
effective  in  causing  flower-fall  than  removal  of  the  stamens  (table  4,  e 
and  /).     On  the  other  hand,  where  half  the  corolla  is  removed  along 
with  the  stamens  fall  occurs  in  larger  buds  than  where  only  the  latter 
organs  are  removed  (table  4,  6). 

7.  Removal  of  only  half  the  corolla  apparently  does  not  induce 
abscission  (table  4,  c). 


408  University  of  California  Publications  in  Botany         [VOL.  5 

8.  Mature  capsules  of  F±  H179  are  apparently  more  sensitive  to 
injury  than  those  of  N.  Langsdorffii  (table  4,  j). 

The  table  dealing  with  the  experiments  on  Lycopersicum  indicates 
that  flowers  of  this  genus  are  remarkably  resistant  to  injury,  fall 
occurring  only  as  the  result  of  stimulation  when  the  ovary  is  injured 
(table  5,  c  and  d).  Since  a  large  number  of  tomato  flowers  are  nor- 
mally abscissed  from  the  different  inflorescences  on  a  plant,  the  sev- 
eral exceptions  to  the  above  statement  noted  in  the  table  probably 
demonstrate  to  what  extent  the  normal  physiological  condition  of  the 
plant  affects  the  matter.  It  seems  to  be  the  opinion  of  most  gardeners 
who  are  familiar  with  the  tomato  plant  that  floral  abscission  in  this 
species  is  more  dependent  upon  soil  conditions  than  upon  injury  or 
sudden  changes  in  climatic  conditions.  It  would  seem,  however,  that 
injuries  to  very  young  fruits  normally  cause  fall,  but  in  this  case  a 
stage  of  development  is  soon  reached  at  which  injury  to  the  berry  has 
no  effect  in  inducing  abscission  (table  5,  /). 

Taking  the  general  results  of  all  the  experiments  into  consideration, 
it  is  seen,  in  the  first  place,  that  where  injury  of  a  certain  type  causes 
fall,  a  stage  of  development  of  the  flower  is  soon  reached  beyond  which 
the  injury  no  longer  causes  fall.  The  increase  in  resistance  to  the 
stimulus  of  mechanical  injury  takes  place  gradually  in  the  species 
investigated,  but  some  of  the  species  are  much  more  resistant  than 
others.  In  the  second  place,  injuries  to  the  ovary  generally  cause 
flower-fall.  Thirdly,  whether  or  not  flower-fall  occurs  as  a  result  of 
injury  to  other  flower  parts  depends  in  some  way  upon  the  quantity 
of  material  removed.  Fourthly,  injury  to  the  pedicel  does  not  cause 
abscission  unless  it  breaks  entirely  the  cellular  connection  between 
flower  and  stem.  Lastly,  it  is  improbable  that  fall  induced  by  injury 
is  due  to  checking  the  transpiration  stream,  since  injury  to  the  ovary 
could  have  no  such  effect.  Also,  a  cut  across  the  pedicel  so  that  the 
flower  hangs  by  only  a  few  tracheae  must  check  transpiration  from  the 
flower  considerably,  yet  in  this  case  no  abscission  occurs. 

It  was  suggested  by  Bequerel  that  injury  might  cause  abscission 
by  checking  the  transpiration  stream  which  passes  up  through  the 
pedicel.  Considerable  doubt  has  already  been  cast  on  this  point  in  the 
above  discussion.  In  order  to  throw  more  light  on  this  question  the 
following  experiment  was  performed  in  an  effort  to  determine  whether 
checking  the  transpiration  stream .  of  itself  and  unaccompanied  by 
mechanical  injury  would  cause  abscission. 

Experiment  12. — As  a  means  of  checking  transpiration  from  the 
flower  a  coating  of  paraffin  seemed  desirable  because  it  hardens 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Sol-ana-ceae    409 

quickly,  thus  permitting  several  coats  to  be  applied.  It  was  doubtful 
whether  other  substances,  such  as  lard,  cocoa  butter  or  vaseline,  which 
might  have  been  used,  would  not  have  been  prevented  from  completely 
covering  the  flower  in  one  coating  by  the  presence  of  numerous  hairs 
and  glandular  fluid  on  the  calyx.  In  this  experiment  flowers  were 
immersed  in  melted  paraffin  to  within  a  millimeter  of  the  separation 
zone  and  allowed  to  stand  in  water  under  normal  atmospheric  condi- 
tions. As  a  test  for  abscission,  the  shoot  was  shaken  or  individual 
flowers  tapped  from  time  to  time.  It  was  found  that  several  Nicotiana 
varieties  and  hybrids  differed  in  their  reaction  to  this  treatment  as 
they  did  in  their  reaction  to  illuminating  gas.  In  AT.  Tabacum  "  Mary- 
land," for  example,  paraffining  the  flowers  failed  to  cause  abscission 
for  six  days,  at  the  end  of  which  time  the  flowers  began  to  fall,  as  did 
those  of  the  control.  Some  varieties,  however,  under  such  treatment, 
throw  off  buds  at  the  end  of  twenty-four  hours,  but  open  flowers  of 
the  same  varieties  are  never  shed.  Whether  or  not  the  buds  fell  in 
these  varieties  depended  largely  on  the  temperature,  at  lower  tempera- 
tures no  fall  occurring.  Also,  in  cases  where  abscission  of  buds  did 
occur  it  was  evident  that  something  was  actually  impeding  the  pro- 
cess ;  none  of  the  white  substance  formed  by  the  isolated  cells  was  seen 
at  the  base  of  the  pedicel  and  the  buds  had  to  be  shaken  or  tapped 
quite  severely  before  they  fell. 

The  results  of  Experiment  12  and  the  various  observations  on  the 
induction  of  abscission  by  mechanical  injury  render  it  extremely 
unlikely  that  checking  the  transpiration  stream  is  ever  a  direct  cause 
of  abscission.  The  few  cases  recorded  above  in  which  such  a  condition 
seems  to  cause  abscission  can  be  better  explained  by  the  action  of  some 
other  factor  than  that  of  interference  with  transpiration. 

In  connection  with  these  experiments  upon  the  effect  of  checking 
transpiration  the  results  of  Lloyd  and  Balls  on  the  effect  of  root 
pruning,  etc.,  in  cotton  must  be  mentioned.  It  was  found  that  a  pre- 
mature shedding  of  flowers  and  young  bolls  followed  root  pruning 
and  further  that,  in  general,  there  is  a  relation  between  boll-shedding 
and  the  rise  and  fall  of  the  water-table.  Proof  positive  is  not  sup- 
plied that  root  pruning  causes  fall  of  flowers  by  reducing  the  water 
supply  of  the  plant  body,  and  any  number  of  other  factors  may  enter 
in  after  such  mutilation  to  bring  about,  in  part  at  least,  such  a  result. 
Experiments  reported  in  the  present  paper  seem  to  leave  no  doubt 
that,  in  Nicotiana  at  least,  temperature  is  a  more  important  factor  in 
controlling  abscission  than  water  supply. 


410  I'nirersity  of  California  Publications  in  Botany         [VOL.."; 

4.  THE  ABILITY  OF  CERTAIN  SPECIES  TO  THROW  OFF  PEDICELS 

FROM  WHICH  ALL  THE  FLORAL  ORGANS  HAVE  BEEN 

REMOVED,  AS  RELATED  TO  THE  INDUCTION  OF 

ABSCISSION  BY  MECHANICAL  INJURY 

It  was  soon  noticed  in  the  experiments  that  all  plants  of  a  species 
in  which  floral  abscission  occurs  throw  off  the  remains  of  the  pedicel 
when  this  organ  is  severed  at  any  point  distal  to  the  separation  layer. 
If  after  such  an  operation  no  abscission  occurs,  it  can  be  safety  con- 
cluded that  floral  abscission  never  occurs  in  that  species.  Petunia 
hybrida,  Salpiglossis  sinuata,  Salpichrora  rhomboidea,  and  Lyciuin 
australis  are  the  only  species  of  the  list  in  table  6  which  do  not  absciss 
flowerless  pedicels  in  this  way.  Nicotiana  Bigelovii,  N.  quadrivalvis, 
and  AT.  multivalvis  occasionally  do  not  throw  off  pedicels  under  such 
conditions.  The  reaction  time  in  cases  where  the  last  three  species  do 
absciss  severed  pedicels  is  very  slow  (four  to  fourteen  days). 

Turning  now  to  the  relation  of  these  observations  to  the  induction 
of  abscission  by  mechanical  injury,  it  is  first  necessary  to  recall  the 
controls  used  in  Experiments  5  and  6  (cf.  pages  399  and  400).  A  fur- 
ther consideration  of  the  reaction  of  these  controls  will  suggest  that 
mechanical  injury  can  induce  abscission  by  the  action  of  the  stimulus 
directly  on  the  cells  in  the  vicinity  of  the  separation  zone.  The  con- 
trol used  in  Experiment  5,  it  will  be  remembered,  showed  that  abscis- 
sion does  not  occur  under  normal  conditions  in  a  series  of  flowers  cut 
as  in  figure  9.  From  the  control  used  in  Experiment  6  it  is  evident 
that  merely  cutting  off  the  flower  at  varying  distances  from  the  sep- 
aration layer,  forming  pieces  as  represented  in  figure  10,  causes  ab- 
scission to  occur,  evidently  as  the  result  of  no  other  stimulus  than 
that  of  severing  the  pedicel.  Now,  if  the  cut  be  made  through  the 
pedicel  at  a  point  approximately  1  mm.  distal  to  the  separation  layer 
in  flowers,  as  represented  on  the  extreme  right  of  figure  9,  abscission 
will  occur  in  the  remaining  piece,  which  is  now  scarcely  2  mm.  in 
length.  It  is  evident  that  the  stimulus  caused  by  severing  the  pedicel 
must  act  directly  on  the  cells  in  close  proximity  to  the  separation 
zone.  Practically  the  same  results  are  obtained  when  the  transverse 
cut  is  made  through  the  base  or  middle  of  the  calyx.  There  is  no 
reason  to  suppose  that  the  stimulus  set  up  by  cutting  through  the 
flower  near  the  base  or  middle  of  the  calyx  differs  in  any  fashion  from 
that  offered  by  a  cut  severing  only  the  pedicel. 


1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    411 

Several  interesting  conclusions  are  brought  out  by  an  examination 
of  the  above  facts.  In  the  first  place,  the  abscission  of  the  remains  of 
severed  pedicels  is  probably  independent  of  the  transportation  of 
materials  from  the  rest  of  the  plant  to  the  separation  zone.  It  may 
result  from  the  action  of  the  stimulus  directly  on  the  cells  in  the 
vicinity  of  the  separation  layer  and  is,  therefore,  largely  independent 
of  such  physiological  processes  as  transpiration  which  might-  conceiv- 
ably enter  in.  In  the  second  place,  abscission  induced  by  mechanical 
injury  is  probably  of  the  same  nature  as  that  of  severed  pedicels  and 
therefore  probably  results  from  the  action  of  the  stimulus  directly  on 
the  cells  in  immediate  proximity  to  the  separation  layer. 


SUMMARY 

The  final  summary  of  results  given  below  is  presented  under 
several  headings  corresponding  to  those  of  the  main  body  of  the 
paper.  Unless  otherwise  stated,  the  results  given  may  be  taken  as 
applying  to  all  the  species  of  the  Solanaceae  in  which  abscission  was 
found  to  occur.  First  is  presented  a  complete  list  of  the  species  which 
were  investigated,  indicating  by  1  those  in  which  floral  abscission 
never  occurs,  by  2  those  in  which  it  very  seldom  occurs,  and  by  3 
those  which  were  actually  examined  microscopically  to  determine  the 
histological  structure  of  the  separation  zone  and  the  method  of 
abscission. 


3  N.    Tabacum  var.   macrophylla 

3  N.  sylvestris 

3  N.  Tabacum  " Maryland" 

3  FJH154    (N.    sylvestris   X   N.    Tab. 

var.  macrophylla) 

3  F,H179    (N.    sylvestris    X    N.    Ta- 
bacum "Cuba") 
3  FjHSG  (N.  sylvestris  X  N.  Tab.  var. 

angustifolia) 
N.  glauca 
3  N.    rustica    (2    varieties — not    bra- 

silia) 

2,  3  N.  Bigelovii  (3  varieties) 
2  N.  quadrivalvis   (2  varieties) 
2  N.  multivalvis 
X.  Sanderae 
N.  rustica  var.  brasilia 
X.  suaveoleiis 


3  Solanum  umbelliferum 

S.  tuberosum 

S.  jasminioides 
3  S.  verbascifolium 

S.  nigrum 

2,  3  lochroma  tuberosa 
3  Oestrum  fasciculatum 

Lycopersicum   esculentum   var.   vul- 

gare 

3  L.  esculentum  var.  pyriforme 
1,  3  Petunia  hybrida 
1,  3  Salpiglossis  sinuata 
3  Datura  sanguineum 
1  Salpichrora  rhomboidea 
1  Lycium  australis 


412  University  of  California  Publications  in  Botany        [VOL.  5 

HISTOLOGY  AND  CYTOLOGY  OF  THE  PEDICEL 

1;  The  separation  layer  arises  in  all  the  species  listed  above,  except 
Lycopersicum  and  Solanum  faiberosum,  at  or  near  the  base  of  the 
pedicel.  In  the  latter  two  species  the  layer  is  located  near  the  middle 
of  the  pedicel,  but  even  in  these  cases,  if  one  considers  the  pedicel  to 
be  composed  of  two  iiiternodes,  the  layer  occurs  at  the  base  of  the 
most  distal  internode. 

2.  The  separation  layer  is  preformed,  ready  to  function  at  any 
stage  in  the  development  of  the  flower  and  represents  (cf.  Kubart's 
first  type,  page  350)   a  portion  of  the  primary  meristem  which  has 
retained  some  of  its  originally  active  condition. 

3.  In  all  the  species  except  Datura  the  separation  cells  are  char- 
acterized by  their  small  size,  isodiametric  shape,  large  amount  of 
protoplasm  and  somewhat  collenchymatous  appearance.     A  study  of 
the  early  histological  development  of  the  pedicel  indicates  that  the 
small  size  of  the  separation  cells  does  not  necessarily  bear  any  relation 
to  abscission.    This  statement  is  supported  by  the  fact  that  in  Datura 
there  is  absolutely  no  visible  difference  between  the  separation  cells 
and  any  other  cells  of  the  pedicel. 

4.  Various  tests  with  stains,  acids,  and  alkalis  fail  to  indicate  any 
chemical  difference  between  the  cell  walls  of  the  separation  cells  and 
the  walls  of  neighboring  cortical  cells  which  do  not  separate.     How- 
ever, the  middle  lamellae  of  cell  walls  in  the  general  region  at  the 
base  of  the  pedicel  seem  somewhat  more  easily  hydrotysed  by  acids 
than  in  the  more  distal  portions. 

5.  A  study  of  the  early  histological  development  of  the  pedicel  in 
Nicotiana  and  Lycopersicum  shows  that  the  grooves  near  which  the 
separation  zone  arises  do  not  necessarily  bear  any  relation  to  abscis- 
sion.    The  grooves  are  formed  because,  in  the  development  of  the 
pedicel,  certain  cells  do  not  increase  in  size  so  fast  as  the  neighboring 
cells  on  either  the  proximal  or  distal  side. 

6.  The  development  of  mechanical  tissue  in  the  pedicel  of  Nicotiana 
continues  through  the  separation  layer,  thus  frequently  holding  the 
fruit  on  the  plant  in  spite  of  the  fact  that  abscission  commonly  occurs 
in  the  cortex.    In  most  of  the  berry-forming  species  of  the  Solanaceae 
this  mechanical  tissue  does  not  become  continuous  through  the  separa- 
tion layer  and  thus  offers  no  impediment  to  fall  when  abscission  occurs 
in  that  region. 


1918]     Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    413 

THE  PROCESS  OF  ABSCISSION 

1.  The  process  of  abscission  conforms  to  the  usual  type,  which 
involves  the  separation  of  cells  along  the  plane  of  the  middle  lamella 
of  the  cell  wall  separating  them. 

2.  No  cell  divisions  or  elongations  were  observed  to  accompany 
abscission. 

3.  All  the  cells  across  the  pedicel  in  the  region  of  the  separation 
layer  take  part  in  separation  except  the  tracheae  and  cuticle,  which 
must  be  broken  mechanically.     The  total  number  of  cells  which  may 
be  involved  is  greater  in  some  species  than  in  others.     This  number 
may  also  vary  in  the  same  species  because  of  changes  in  the  external 
conditions. 

4.  Cell  separation  is  brought  about  by  the  hydrolysis  and  conse- 
quent dissolution  of  the  middle  lamella  (primary  cell  membrane)  or 
perhaps  both  the  primary  and,  in  part,  secondary  cell  membranes. 
The  agency  active  in  the  hydrolysis  of  the  cell  membranes  is  probably 
an  enzyme. 

5.  An  increase  in  cell  turgor  frequently  occurs  during  abscission, 
but  probably  serves  merely  to  hasten  and  facilitate  the  process.    Most 
of  the  frequently  observed  expansion  and  the  turgid  appearance  of 
the  separation  cells  during  abscission  are  probably  due  to  the  natural 
release  of  pressure  caused  by  the  dissolution  of  the  middle  lamellae. 

6 :  Abscission  of  the  style  and  corolla  in  Nicotiana  and  Datura 
resembles,  to  a  large  extent,  abscission  of  the  flower. 

TIME  OF  ABSCISSION 

1.  The  length  of  time  between  anthesis  and  normal  flower-fall  due 
to  lack  of  fertilization  differs  among  the  varieties  of  Nicotiana.    This 
variation  was  found  to  range  between  an  average  of  five  to  eighteen 
days  in  some  fifteen  species  and  varieties  of  Nicotiana.     A  much 
smaller  range  of  variation   (0.7  to  four  days,  with  the  largest  fre- 
quency in  the  three  day  group)  was  noted  for  the  time  between  an- 
thesis and  fall  of  the  corolla  after  pollination. 

2.  The  stimulation  of  the  stylar  tissues  by  the  growth  of  the  pollen 
tubes  tends  to  shorten  the  time  between  anthesis  and   fall   of  the 
corolla,  this  effect  being  independent  of  fertilization.     Such  stimula- 
tion of  the  stylar  tissues  has  no  appreciable  effect  upon  floral  ab- 
scission. 

3.  Floral  abscission  occurs  in  Fx  H179  seven  hours  after  subjecting 
shoots  of  the  plant  to  1.5  per  cent  illuminating  gas  at  a  temperature 


414  University  of  California  Publications  in  Botant/         [VOL.  3 

of  19°  C.  It  occurs  in  Nicotiana  Tabacum  "Maryland"  in  eight  hours 
under  the  same  conditions.  The  actual  time  involved  in  the  process  of 
cell  separation  in  the  above-mentioned  cases  lies  within  thirty  to  forty 
minutes  in  the  hybrid  and  within  forty-five  to  sixty  minutes  in  the 
Tabacum  variety.  Normal  abscission  in  these  forms  is  much  slower 

4.  The  length  of  the  reaction  time  in  cases  of  flower-fall  due  to 
mechanical  injury  shows  that  this  length  of  time  depends  more  on 
the  age  of  the  flower  than  on  the  type  of  injury. 

5.  Temperature  is  the  most  important  conditioning  factor  in  esti- 
mates of  the  time  of  abscission. 

EXPERIMENTAL  INDUCTION  OF  ABSCISSION 

1.  Floral  abscission  is  induced,  in  a  large  number  of  the  species 
investigated,  by  illuminating  gas  or  laboratory  air.     The  increase  in 
resistance  to  abscission  stimulated  in  this  manner  takes  place  suddenly 
in  some  species,  since  abscission  will  not  occur  after  the  opening  of 
the  corolla.    In  other  species  this  condition  does  not  exist. 

2.  It  is  possible  to  induce  the  process  of  abscission  with  illuminat- 
ing gas  in  small  isolated  pieces  of  the  pedicels  or  in  longitudinal  sec- 
tions of  the  pedicel  cut  free-hand  from  fresh  material. 

3.  Abscission  in  Nicotiana  and  Lycopersicum  is  induced  by  certain 
types  of  severe  injury  and  not  by  others.    Injury  to  the  ovary  seems 
more  effective  in  causing  abscission  than  injury  to  other  parts  of  the 
flower.    In  the  case  of  these  other  flower  parts,  it  seems  necessary  that 
a  certain  amount  of  tissue  be  actually  removed  or  destroyed  before 
fall  occurs.    Injury  to  the  pedicel  does  not  cause  abscission  unless  it 
breaks    entirely    the    connection    between    floral    organs    and    stem. 
Flower-fall  in  Lycopersicum  is  not  readily  induced  by  injury.    Floral 
abscission  in  this  genus  is  more  dependent  upon  physiological  condi- 
tions brought  on  by  abnormal  soil  conditions. 

4.  Experiments  on  the  induction  of  abscission  in  small  isolated 
pieces  and  in  flowers  with  only  a  small  portion  of  the  stem  proximal 
to  the  separation  layer  attached  indicate  that  the  stimulus  produced 
by  the  action  of  external  factors  such  as  illuminating  gas  and  mechan- 
ical injury  can  cause  abscission  by  acting  directly  on  the  cells  in  close 
proximity  to  the  separation  zone.     The  action  of  external  factors  is 
thus  largely  independent  of  such  physiological  processes  as  transpira- 
tion which  might  enter  in.     This  statement  is  supported  by  experi- 
ments  which    show   that    abscission   is   not    necessarily    induced    by 
checking  transpiration  from  the  flower. 


1918]     Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    415 

CONCLUSION 

It  is  proposed  in  what  follows  to  take  up  consideration  of  such 
phenomena  in  connection  with  abscission  as  are  still  but  slightly 
understood.  One  of  the  most  perplexing  of  these  is  undoubtedly  the 
definitely  predetermined  location  of  the  separation  layer  _when  no 
morphological  and  sometimes  no  physiological  (Datura)  difference  can 
be  detected  between  the  cells  that  separate  and  those  that  do  not. 
There  need  be  no  doubt,  however,  that  such  a  difference  does  exist 
and  that  a  sufficient  refinement  of  technique  will  serve  to  detect  it. 

In  considering  this  matter  further  it  may  be  recalled  that  the 
separation  layer  in  axial  abscission  is  located  at  or  near  the  base  of  an 
internode.  There  is  undoubtedly  some  connection  between  this  fact 
and  the  fact  that  the  cells  most  active  physiologically  are  often  found 
in  this  region.  The  growth  of  an  internode  may  be  brought  about  by 
the  action  of  an  intercalary  meristem  located  at  the  base  of  the  organ 
and  a  meristem  so  located  in  some  cases  retains  its  original  activity 
in  the  mature  internode.  Now  it  is  well  known  that  the  walls  of  young 
active  cells  are  more  readily  subject  to  hydrolysis  than  the  walls  of 
older  cells,  because  of  the  fact  that  the  former  contain  more  water. 
If  we  assume,  then,  that  the  internode  is  a  metabolic  gradient  with 
the  most  active  cells  at  the  base,  it  would  be  expected  that  the  walls 
of  these  cells  would  be  more  subject  to  hydrolysis  than  any  other  cells 
of  the  internode.  If  some  hydrolysing  agency  becomes  active 
throughout  the  pedicel,  it  might  be  expected  that  the  walls  of  the 
cells  at  the  base  of  the  internode  would  react  first,  causing  their  sep- 
aration and  thus  cutting  off  the  flower  or  internode.  By  assuming 
in  this  way  that  separation  always  takes  place  through  the  most 
active  cells  of  the  internode  it  seems  possible  to  explain  the  predeter- 
mined location  of  the  separation  layer. 

There  is  undoubtedly  some  connection  between  the  above  problem 
and  the  fact  that  some  plants  must  perfect  a  separation  layer  before 
detachment  can  take  place.  In  such  cases  the  tissues  at  the  base  of 
the  organ  are  too  old  for  separation.  The  same  stimulus  which  causes 
abscission  in  some  species  causes  a  renewal  of  activity  at  the  basal 
region  of  an  organ,  resulting  in  cell  divisions  and  new  cells.  These 
new  cells  may,  under  a  continuation  of  the  stimulus,  separate  one 
from  another. 

Another  perplexing  problem,  which  also  includes  many  subsidiary 
problems,  relates  to  the  exact  course  taken  by  the  stimuli  in  causing 


416  University  of  California  Publications  in  Bota-ny        [VOL.  5 

abscission.  Experiments  described  in  the  present  paper  have  indi- 
cated that  this  course  may  be  direct  as  well  as  indirect.  Assuming  for 
the  present  that  some  of  the  factors  bringing  about  abscission  always 
act  directly  while  others  act  indirectly,  we  might  classify  the  general 
factors  operative  in  the  case  of  the  Solanaceae  as  follows : 

DIRECT       1.  Narcotic  vapors. 

2.  Injury  to  floral  organs. 

3.  Sudden  rise  in  temperature. 

4.  Lack  of  fertilization. 
INDIRECT  5.  Changes  in  soil  conditions. 

6.  Factors  evident  in  normal  physiological  development. 

The  direct  factors  act  directly  on  the  cells  at  the  base  of  the  pedicel 
and  consequently  the  reaction  time  must  be  comparatively  rapid.  The 
indirect  factors  act  indirectly  through  the  general  physiological  con- 
dition, which  in  turn  furnishes  the  direct  stimulus  for  cell  separation. 
In  the  latter  case  the  reaction  time  must,  as  a  general  rule,  be  slow. 
The  nature  of  factors  under  6  are  most  difficult  to  understand.  An 
example  of  the  action  of  these  factors  would  be  given  in  those  cases 
where  most  of  the  flowers  of  an  inflorescence  are  normally  abscissed 
leaving  only  one  or  two  to  continue  development,  and  in  those  species 
which  absciss  male  flowers  after  anthesis. 

A  further  analysis  of  the  course  of  the  abscission  reaction  intro- 
duces another  unsettled  problem — the  nature  of  the  agency  which  is 
directly  responsible  for  the  dissolution  of  the  middle  lamella.  It  has 
been  pointed  out  before  that  an  enzymatic  body  of  some  kind  is  prob- 
ably involved.  The  following  discussion  brings  out  certain  facts 
which  it  is  necessary  to  take  into  consideration  when  speculating  as 
to  the  nature  of  this  supposed  enzyme.  The  activity  of  the  enzymatic 
body  must  be  subject  to  both  internal  and  external  conditions.  The 
enzymatic  material  must  also  be  extremely  sensitive  to  slight  changes 
in  the  normal  environment.  It  must  be  continually  present  in  the 
cells  of  the  separation  zone  and  ready  at  any  moment  to  react  to  such 
changes  in  the  environment.  A  comparison  of  several  species  in 
regard  to  their  abscission  reactions  to  the  factors  listed  above  indicates 
that  this  supposed  enzyme  must  be  more  sensitive  in  some  species 
than  in  others.  Indeed,  in  certain  species  in  which  no  abscission 
occurs  the  enzyme  must  be  absent  from  the  region  of  the  separation 
zone  or  entirely  inactive.  Finally,  it  seems  necessary  to  assume  that 
in  certain  species  the  action  of  the  enzyme  is  suddenly  inhibited  at 
about  the  time  of  the  opening  of  the  corolla. 


1918J     Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    417 

It  has  been  noticed  in  all  the  experiments  detailed  above  that 
older  flowers  are  less  subject  to  "spontaneous"-  abscission  than 
younger  ones.  The  transition  line  as  to  size  or  age  beyond  which 
no  abscission  occurs  can  not  in  most  cases  be  definitely  drawn;  that 
is  to  say,  the  development  of  a  resistance  to  stimuli  takes  place  grad- 
ually. This  is  probably  explained  by  the  fact  that  cell  walls  gradually 
become  less  subject  to  hydrolysis  with  age.  The  celluloses— a»d  pec- 
toses  lose  water  with  age  and  it  is  well  known  that  these  compounds  are 
subject  to  hydrolysis  in  proportion  to  the  amount  of  water  they  con- 
tain. In  those  cases  where  the  increase  in  resistance  to  stimuli  takes 
place  suddenly  it  is  necessary,  as  suggested  above,  to  assume  some 
kind  of  inhibitor  of  the  enzymatic  action. 

The  effect  that  pollination  has  in  hastening  abscission  of  the 
corolla  is  a  subject  which  is  related  to  the  phenomena  described  by 
Fitting  (1909)  for  orchids.  The  phenomena  are  as  yet  only  slightly 
understood.  The  explanation  seems  to  involve  some  relaying  of 
stimulus  from  cell  to  cell.  This  is  also  involved  in  the  explanation  of 
floral  abscission  induced  by  injury  to  the  ovary.  These  two  cases 
and  others  indicate  that  in  some  instances,  at  least,  abscission  responses 
are  related  to  tropistic  responses  as  Fitting  (1911)  has  suggested. 

Finally,  attention  may  be  called  to  the  fact  that  the  most  pressing 
need  in  connection  with  all  the  problems  mentioned  above  is,  in  the 
first  place,  to  establish  by  some  experimental  means  a  definite  connec- 
tion between  some  enzymatic  body  and  the  process  of  abscission  and, 
in  the  second  place,  more  definite  knowledge  as  to  the  role  which  cell 
turgor  plays  in  cell  separation.  Taking  all  the  facts  into  considera- 
tion, it  is  evident  that  abscission  is  fundamentally  a  physiological 
problem,  the  crux  of  which  lies,  as  in  all  such  problems,  in  the  bio- 
chemistry of  the  cell. 

The  studies  reported  upon  above  were  carried  on  under  the  direc- 
tion and  supervision  of  Professor  T.  H.  Goodspeed  and  I  am  under 
deep  obligation  to  Professor  F.  E.  Lloyd  for  many  valuable  sugges- 
tions both  throughout  the  course  of  the  experiments  and  during  the 
preparation  of  this  report  of  them. 


418  University  of  California  Publications  in  Botany        [VOL.  5 


LITEEATUEE  CITED 

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1916.     Some  recent  researches  in  plant  physiology,  p.  64. 

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1907.  Sur  un  cas  remarquable  de  autotomie  de  pedoncle  floral  de  tabac 
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BROWN,  H.  T.,  and  ESCOMB,  F. 

1902.  The  influence  of  varying  amounts  of  carbon  dioxide  in  the  air  on 
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1899.     Vermehrung  der  Laubmoose.  Jena,  1899.    Quoted  from  Lloyd  (1914a). 

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1915.  Phenomenon  of  self -sterility.     Am.  Nat.,  vol.  49,  p.  77. 

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1914a.  Abscission  in  flowers,  fruits  and  leaves.     Ottawa  Nat.,  1914. 

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of  plants,  1914,  p.  72. 

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1918]    Kendall:  Abscission  of  Flowers  and  Fruits  in  Solanaceae    419 

LEE,  E. 

1911.     Morphology  of  leaf -fall.     Ann.  Bot.,  vol.  25,  p.  51. 

LOEWI,  E. 

1907.  Blattablossung    und    verwandte    Erscheinungen.      Proc.    Akad.    Wien, 

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1860.     Ueber  den  Ablosungsprozess  saftiger  Pflanzenorgane.     Bot.  Zeit.,  vol. 

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1885.  Ueber  anatomische  Veranderungen  welche  in  den  perianthkreisen  der 
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1908.  Ueber    Turgorsteigerung    in    der    Atmospher    von    Narkotica.      Lotos, 

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EICHTEE,  O.,  and  GRAFE,  V. 

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1913.     Das  botanisehe  Praktikum,  p.  349. 
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PLATE  49 

Fig.  1.  Base  of  pedicel  of  Nicotiana  bud  showing  groove,  separation  zone, 
and  process  of  abscission  well  under  way  in  dorsal  cortex. 

Fig.  2.  Portion  of  cortex  in  the  separation  layer  of  Nicotiana  showing  the 
bulging  of  the  epidermis,  one  of  the  first  signs  of  abscission. 


[420] 


UNIV.   CALIF.    PUBL.    BOT.    VOL.   5 


[KENDALL]    PLATE  49 


Fig.  1 


Fig.  2 


PLATE  50 

Fig.  1.  Portion  of  the  base  of  the  pedicel  of  Nicotiana  at  a  late  stage  in 
the  process  of  abscission  showing  the  independent  origin  of  the  process  in  the 
pith. 

Fig.  2.  Portion  of  the  cortex  in  the  separation  layer  of  Nicotiana  showing- 
separating  cells  next  to  the  vascular  system. 


[422] 


UNIV.  CALIF.    PUBL.    BOT.   VOL.  5 


[KENDALL]    PLATE  50 


Fig.  1 


• 


* 


Fig.  2 


PLATE  51 

Portion  of  the  separation  layer  of  Nicotiana  showing  cells  in  the  process  of 
separation  in  the  upper  part  of  the  section. 


I  424  ] 


UNIV.   CALIF.    PUBL.    BOT.    VOL.   5 


[KENDALL]    PLATE  51 


PLATE  52 

Fig.  1.  Portion  of  dorsal  cortex  near  the  groove  in  the  pedicel  of  \icot nnm, 
showing  the  abscission  process  well  under  way. 

Fig.  2.  Group  of  isolated  cells  washed  off  from  end  of  a  freshly  abscissed 
pedicel  of  Nicotiana. 

Fig.  3.  Single  isolated  cell  showing  the  thinness  of  the  remaining  cell 
membrane. 


42(5  ] 


UNIV.    CALIF.    PUBL.    BOT.    VOL.   5 


[KENDALL]    PLATE 


- 


i 


'\ 

Fig.  ] 


' 


Fig.  3 


PLATE  53 

Fig.  1.  Portion  of  pedicel  of  Lycopersicum,  showing  groove  and  separation 
zone. 

Fig.  2.  Portion  of  cortex  of  pedicel  of  Lycopersicum,  showing  groove  and 
abscission  process  fairly  well  along;  cell  separation  first  takes  place  between 
only  two  tiers  of  cells  before  spreading  to  others. 


[ 428  ] 


UNIV.  CALIF.    PUBL.    BOT.   VOL.  5 


[KENDALL]    PLATE  53 


i 


Fig.  2 


UNTVESSITY  OF  CALIFORNIA  PUBLICATIONS— (Continued) 

6.  Contributions  to  the  Knowledge  of  the  California  Species  of  Crusta- 
ceons  Corallines.  EL   by  Maurice   B&rstow   Nichols.     Pp.   549-870; 

plates  10-13.    April,  1909 _ . _      Jfi 

1.  New  Chloropbyceae  from  California,  by  Nathaniel  Lyon  Gardner.    Pp. 

371-375;  plate  14.     April,  1909   J.O 

8.  Plantae  Mexicanae  Pnrpusianae,  I,  by  T.  S.  Brandegee.    Pp.  377-396. 

May,  1909  „ ;. .15 

Index,  pp.  397-400. 
VOL  *.     1010-1912. 

1.  Studies  in  Ornamental  Trees  and  Shrub*,  by  Harvey  Monroe  HalL    Pp. 

1-74;  plates  1-11;  15  text-figures.    March,  1910  < _      .78 

2.  Gracilariopkila,  a  New  Parasite  on  Gractlaria  confervoidcs,  by  Harriet 

L.  Wilson.  Pp.  75-84;  plates  12-13.    May,  1910  ....;~-,--,..^ _      J.O 

3.  Plantae  Mexicanae  Purpuaianae,  II,  by  T.  S.  Brandegee.     Pp.  85-95, 

May,  1910  - - —      JLO 

4.  Leuvenia,  a  New  Genus  of  Flagellates,  by  N.  L.  Gardner.    Pp.  97-106; 

plate  14.    May,  1910 „. „ — „ ,10 

6.  The  Genus  Sphaerosoma,  by   William  Albert  SetchelL    Pp.  107-120; 

plate  15.     May,  1910 „ -.      ,15 

6.  Variations  in  Nuclear  Extrusion  Among  the  Fueaceae,  by  Nathaniel 

Lyon  Gardner.    Pp.  121-136;  plates  16-17.    August,  1910 . . „      J.8 

7.  The  Nature  of  the  Carpostomes  in.  the  Cystocarp  of  Ahnfeldtia  ffigarti- 

noides,  by  Ada  Sara  McFadden.    Pp.  137-142;  plate  18.    February, 
1911 .05 

8.  On  a  Colacodasya  from  Southern  California,  by  Mabel  Effie  McFadden, 

Pp.  143-150;  plate  19.     February,  1911  . „ .08 

9.  Fructification  of  Macrocystis,  by  Edna  Juanita  Hoffman,    Pp.  151-158; 

plate  20.     February,  1911 . „ .05 

10.  Erythrophyllum  dclesserioides  J.  Ag.,  by  Wilfred  Charles  Twlss.     Pp. 

159-176;  plates  21-24.     March,  1911 .15 

11.  Plantae  Mexicanae  Purpusianae,  HI,  by  T.  S.  Brandegee.    Pp.  177-194. 

July,  191 1  ._.. -.-..„ _ .._ —      J.5 

12.  New  and  Noteworthy  Calif orniaa  Plants,  L  by  Harvey  Monroe  HalL 

Pp.  195-208.     March,   1912 _..:.' .15 

IS.  Die  Hydrophyllaceen  der  Sierra  Nevada,  by  August  Brand,    Pp.  209- 

227.     March,  1912  -.-  . . - ~ - — .20 

14.  Algae  Novae  et  Minus  Cognitae,  I,  by  William  Albert  Setchell.    Pp. 

229-268;  plates  25-31.     May,  1912— —       .40 

15.  Plautae  Mexicanae  Purpusianae,  IV,  by  Townshend  Stith  Brandegee. 

Pp.  269-281.     June,  1912  ~ ...._.„..        .19 

16.  Comparative  Development   of  the   Cystocarps  of  Antitlmmnion   and 

Prionitis,  by  Lyman  Luther  Daines.      Pp.  283-302;   plates  32-34. 
March,  1913  „ ...: _ .20 

17.  Fungus  Galla  on  Cystoseira  and  Halidrys.  by  Lulu  May  Estee.    Pp.  305- 

316;  plate  35.     March,  1913 _      .10 

18.  New  Fueaceae,  by  Nathaniel  Lyon  Gardner.    Pp.  317-374;  plates  36- 

53.     April,  1913 .75 

19.  Plantae  Mexicanae  Purpusianae,  V,  by  Townshend  Stith  Brandegee. 

Pp.  375-388.    June,  1913 .15 

Index,  pp.  389-397. 
VoL  §.    1912-. 

1.  Studies  in  Ntcoiiana,  I,  by  William  Albert  SetcheU.    Pp.  1-86.     De- 

cember,  1912 -      1.25 

2.  Quantitative  Studies  of  Inheritance  in  Nicotiana  Hybrids,  I,  by  Thomaa 

Harper  Gobdspeed.    Pp.  87-168.    December,  1912  _      1.00 

5.  Quantitative   Studies   of   Inheritance   in   Nicotiana   Hybrids,   n,   by 

Thomas  Harper  Goodspeed.     Pp.  169-188.    January,  1913 £0 

4.  On  the  Partial  Sterility  6f  Nicotiana  Hybrids  made  with  N.  Sylveslris 

as  a  Parent,  by  Thomas  Harper  Goodspeed.    Pp.  189-198.    March, 

1913 ao 

5.  Notes  on  the  Germination  of  Tobacco  Seed,  I,  by  Thomas  Harper  Good- 

speed.     Pp.  199-222.     May,  1913 -       .25 

6.  Quantitative  Studies  of  Inheritance  in  Nicotiana  Hybrids,  HI,  by 

Thomas  Harper  Goodspeed.    Pp.  223-231.    April,  1915 _„       »10 

7.  Notes  on  the  Germination  of  Tobacco   Seed,   II,  by  Thomas  Harper 

Goodspeed.     Pp.  233-248.     June,  1915  -        .15 

8.  Parthenogenesis,   Parthenocarpy   and   Phenospermy  in  Nicotiana,  by 

Thomas  Harper  Goodspeed.    Pp.  249-272,  plate  35.    July,  1915.. —       .25 


UNIVERSITY  OF  CALIFORNIA  PUBLICATIONS— (Continued) 

9.  On  the  Partial  Sterility  of  Nicotiana  Hybrids  made  with  N,  sylvestris 
as  a  Parent.  II,  by  T.  H.  Goodspeed  and  A.  H.  Ayres.  Pp.  273-292, 
plate  36.  October,  1916  .20 

10.  On  the  Partial  Sterility  of  Nicotiana  Hybrids  made  with  N.  sylvestris 

as  a  Parent.  III.  An  Account  of  the  Mode  of  Floral  Abscission  in 
the  F,  Species  Hybrids,  by  T.  H.  Goodspeed  and  J.  N.  KendalL 
Pp.  293-299.  November,  1916  05 

11.  The  Nature  of  the  T^  Species  Hybrids  between  Nicotiana  sylvestris 

and  Varieties  of  Nicotiana  tabacum,  with  Special  Reference  to  the 
Conception  of  Reaction  System  Contrasts  in  Heredity,  by  T.  H. 
Goodspeed  and  R.  E.  Clausen.  Pp.  301-346,  plates  37-48.  Janu- 
ary, 1917  46 

12.  Abscission  of  Flowers  and  Fruits  in   the   Solanaceae,   with   Special 

Reference  to  Nicotiana,  by  John  N.  Kendall.    Pp.  347-428,  10  text 

figures,  plates  49-53.     March,  1918  .85 

VoL  6.    1914- 

1.  Parasitic  Florideae,  X,  by  William  Albert  SetcheU.    Pp.  1-34,  platei 

1-6.    April,  1914 .35 

2.  Phytomorula  regularis,  a  Symmetrical  Protophyte  Related  to  Coelat- 

trum,  by  Charles  Atwood  Kofoid.    Pp.  35-40,  plate  7,    April,  1914.       .05 
8.  Variation  in  Oenothera  ovata,  by  Katherine  Layne  Brandegee.    Pp.  41- 

60,  plates  8-9.    June,  1914  .„._ „ J.O 

4.  Plantae  Mexicanae  Purpusianae,  VI,  by  Townshend  Stith  Brandegee. 

Pp.  51-77.     July,  1914 „ .25 


6.  The  Scinaia  Assemblage,  by  WiUiam  Albert  SetchelL     Pp.  79-162, 

platfe*  10-16.     October,  1914  .7B 

6.  Notes  on  Pacific  Coast  Algae.     L  Fylaiella  Postelsiae,  n.  sp.,  a  New 

Type  In  the  Genus  Pylaiella,  by  Carl  Skottsberg.    Pp.  153-164,  plates 
17-19.     May,   1915  .15 

7.  New  and  Noteworthy  Californian  Plants,  n,  by  Harvey  Monroe  Hall. 

Pp.  165-176,  plate  20.    October,  1915  .15 

8.  Plantae  Mexicanae  Purpusianae  VII,  by  Townshend  Stith  Brandegee. 

Pp.  177-197.    October,  1915  _ _       .25 

9.  Floral  Relations  Among  the  Galapagos  Islands,  by  A.  L.  Kroeber. 

Pp.  199-220.    March,  1916 .20 

10.  The  Comparative  Histology  of  Certain   Californian   Boletaceae,  by 

Harry  S.  Yates.    Pp.  221-274,  plates  21-25.    February,  1916 J50 

11.  A  Revision  of  the  Tuberales  of  California,  by  Helen  Margaret  Gilkey. 

Pp,  275-356,  plates  26-30.     March,  1916  -       .80 

12.  Species  Novae  vel  Minus  Cognitae,  by  T.  S.  Brandegee.    Pp.  357-361. 

May,  1916 _ .05 

13.  Plantae  Mexicanae  Purpusianae,  Vni,  by  Townshend  Stith  Brandegee. 

Pp.  363-375.    March,  1917 15 

14.  New  Pacific  Coast  Marine  Algae,  I,  by  Nathaniel  Lyon  Gardner,    Pp. 

377-416,  plates  31-35.    June,  1917 .40 

16.  An  Account  of  the  Mode  of  Foliar  Abscission  in  Citrus,  by  Robert  W. 

Hodgson.    Pp.  417-428,  3  text  figures.    February,  1918 10 

VoL  7.    1916- 

1.  Notes  on  the  Californian  Species  of  Trillium  L.    I.  A  Report  of  the 

General  Results  of  Field  and  Garden  Studies,  1911-1916,  by  Thomas 
Harper  Goodspeed  and  Robert  Percy  Brandt.  Pp.  1-24,  plates  1-4, 
October,  1916 .25 

2.  Notes  on  the  Californian  Species  of  Trillium  L.    IL  The  Nature  and 

Occurrence  of  Undeveloped  Flowers,  by  Thomas  Harper  Goodspeed 

and  Robert  Percy  Brandt.  Pp.  25-38,  plates  5-6.  October,  1916  _  J.5 

S.  Notes  on  the  Californian  Species  of  Trillium  L.  III.  Seasonal  Changes 
in  Trillium  Species  with  Special  Reference  to  the  Reproductive  Tis- 
sues, by  Robert  Percy  Brandt.  Pp.  39-68,  plates  7-10.  December, 
1916  „ .SO 

4.  Notes  on  the  Californian  Species  of  Trillium  L.  IV.  Teratological 
Variations  of  Trillium  sessile  var.  giganteum  H.  &  A.,  by  Thomas 
Harper  Goodspeed.  Pp.  69-100,  plates  11-17.  January,  1917 -90 


NON-CIRCULATING  BO 


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UNIVERSITY  OF  CAUFORNIA  UB1 


